Develop Reference

Background Recyclerview shows incorrectly after changing to Light/ Dark theme from Android App

Anyone can help me?
I'm developing the Light and Dark theme function in the android app, everything goes fine, but the background of Recyclerview shows incorrectly.
The root background of xml file is White color and I didn't set background for recyclerview in xml. However, after changing from Light to Dark or Dark to Right=> background of recyclerview automatically change to a strange color(this color didn't see in my color.xml resource). I tried to set background of Recyclerview to #null or transparent in code and xml file as well but the background of recyclerview didn't remove that strange color.
So anyone knows exactly the reason why, please help me and much appreciated. Thanks
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I am guessing you are using the DayNight Theme. If so you should have to themes.xml folder in res, one is called as mentioned and the other is with the extension (night). There you can define a color in both xml files. It has the same name but different color values like this:
<style name="AppTheme" parent="Theme.AppCompat.DayNight">
<item name="colorPrimaryDark">#ffffff</item> <!-- you normally shouldn't hardcode color -->
</style>
The same for your folder (night):
<style name="AppTheme" parent="Theme.AppCompat.DayNight">
<item name="colorPrimaryDark">#000000</item> <!-- you normally shouldn't hardcode color -->
</style>
The idea is to have one attribute name that contains 2 colors and takes the correct one if needed. For futher understanding I suggest to take a look at this reference to get more familiar with Themes and Styles. Now you set up your Day and Night files properly, you can implement it by using it in your recyclerView as follows:
android:background="?attr/colorPrimaryDark"
Another tip is to make custom colors in multiple colors.xml files to make them for more unique use. In this case colorPrimaryDark effect your whole app. It is also suggested to modify layouts and the visuals of widgets to take effect only on those. (e.x. your recyclerView). In my app I used colorPrimaryDark for all Background (that should be same for more clean design). I think you get the keypoint. Take a look around the net and this forum and you will find your final design strategy. Cheers! :)

How come the color I set for cardBackgroundColor isn't exactly the one that gets shown?

Background
I'm trying to prepare dark theme for an app, and one of the requirements is to have a specific color for cards and dialogs : #ff3c454c
The problem
Whether I set it by force ( app:cardBackgroundColor="#3c454c") , by reference ( app:cardBackgroundColor="#color/...) or just in the theme - in all cases I don't get the color I've set. Instead I get the color of #525A61.
I've tested just a red color (#f00) just to be sure it affects the card, and it does, and for this color it indeed gets to be fine. But for the color I've set, it doesn't.
What I've tried
As I wrote, I tried multiple ways to set the color. In the beginning I wanted to use just the theme itself, so I've set it as such:
styles.xml
<style name="AppTheme" parent="Theme.MaterialComponents.DayNight.DarkActionBar">
...
<item name="colorBackgroundFloating">#color/colorBackgroundFloating</item>
<item name="colorSurface">#color/colorBackgroundFloating</item>
</style>
res/values-night/colors.xml
<color name="colorBackgroundFloating">#ff3c454c</color>
Later I tried to use the color directly and even set it as hard coded. Still got the incorrect color when it gets shown.
Seeing this could be a bug on the library itself, I've reported about this here (include a sample project, if you wish to check it out).
I've noticed the exact same issue occurs for BottomNavigationView and probably other similar cases.
The question
Why does it occur?
Is there any workaround for this?
Something that will fix it globally for all views that use these attributes ?

What you are seeing is the elevation overlay that they introduced to make elevation more noticeable while in Dark Theme, where shadows are not so visible. You can read about it here : https://material.io/develop/android/theming/dark/ in the section "Elevation Overlays"
The simple solution if you don't want this behavior is to add this to your theme.
<item name="elevationOverlayEnabled">false</item>
And you can also adjust it to another color or even a more subtle version of the overlay by changing the alpha:
<item name="elevationOverlayColor">#80FFFFFF</item>
EDIT with more info from https://github.com/material-components/material-components-android/issues/1133
If you want to disable it for only one component or widget, you can define a style for it with a theme overlay and use it in a specific layout:
<style name="ThemeOverlay.MyApp.ElevationOverlayDisabled" parent="">
<item name="elevationOverlayEnabled">false</item>
</style>
<style name="Widget.MyApp.CardView" parent="Widget.MaterialComponents.CardView">
<item name="materialThemeOverlay">#style/ThemeOverlay.MyApp.ElevationOverlayDisabled</item>
</style>
And if you want to disable it for all cards in your app but keep it in other components you can set that style as the default style for material card views:
# Set in your app theme
<item name="materialCardViewStyle">#style/Widget.MyApp.CardView</item>

Unexpected screen gradient in ListView

I have a ListView with some rows and a custom checkbox on the right hand side. On my OS 4.4 Nexus 4 it seems like a gentle gradient is being applied to the list row backgrounds, creating an ugly artifact on the checkboxes (they disappear half way down, and then invert for the bottom half). On other devices I don't see this problem, and I also don't see it in an OS 4.4.2 emulator.
I haven't been able to find any information online about this, so I'm not sure if it's specific to the device, or the exact OS flavor.
Is this something I can disable? If not, what advice should I give my asset designer?
Here's a screenshot:

The Holo.Light theme uses a subtle grey gradient background. It might only be more apparent on one of your devices due to the screen's contrast/brightness.
You can just set the background to solid white by using the android:windowBackground tag in your Activity's theme:
<style name="SolidWhiteTheme" parent="android:Theme.Holo.Light">
<item name="android:windowBackground">#android:color/white</item>
<item name="android:colorBackground">#android:color/white</item>
</style>
And then applying the theme to your activity in your AndroidManifest.xml like so:
<activity
android:theme="#style/SolidWhiteTheme"
...
>
...
</activity>

As Denley mentioned, the reason for this is the default background specified by the Holo.Light theme.
However, since this background is specifically affecting your ListView, I would suggest setting the background of your ListView in your xml file. Code below.
<ListView
android:background="#android:color/white">

Try specifying a background color for your activity (android:background). If the background is not explicitly set, the device may be using it's own device-specific default background, which is why the gradient is only shown on your Nexus 4 and not other devices.

change style attribute at runtime

I set the application theme using the following style
<style name="Background1" parent="#android:style/Theme.NoTitleBar.Fullscreen">
<item name="android:windowBackground">#drawable/bigl_2</item>
<item name="android:windowNoTitle">true</item>
</style>
using this method :
setTheme(R.style.background1);
Here my requirement is I want to change the theme with a bitmap image, how can I do it at runtime?
Thanks

You may want to have a look at the same question: android dynamically change style at runtime
As it seems to me, the matter is that generic style attributes can force screen re-layouting, so there is no API to change styles at runtime - you need to create screen anew (the same approach is dictated for landscape/portrait screen orientation changes, where Activity is re-created and large brains of Android system architects invented savedState in onCreate() to easy (complicate?) that process, and then invented Fragments to easy that even more))
So, changing styles generally means changing layout. But you can change Theme attributes at runtime (per the link above - I haven't tried myself), and it will work since it doesn't need re-layouting.
As a workaround, since you will need to recreate views for new layout, you can simplify code by applying different Theme, see the link.

Android preference title/summary text style

I have a couple custom preference items -- one that displays a swatch of the currently selected color, and another one that displays a thumbnail.
I have a custom layout for these that matches up very well, and have found that I can make the text appearance match by using android:textAppearance="?android:attr/textAppearanceLarge" as part of the TextView's xml. The problem is, while these generally look fine, they must not be the appearance the 'official' preferences use, since the colors end up wrong on some devices. In particular I'm porting my application to the Nook Color, and it uses a light grey background and black text color on the preference screen instead of black background/light grey text. My text color in this situation stays the same, but the rest of my layout is themed appropriately.
I'm really unsure what I'm supposed to do here to make my text match up with the 'official' theme. Should I be using obtainStyledAttributes and running though my layout to set things? The tutorials I've seen on using that so far have been really baffling, and it seems like there must be a textAppearance or style I can set in the XML to fix this.

You've to define your own application theme which inherits from the official theme you want. Actually, you can just define a theme MyTheme extending the Theme.Light for example.
Create an res/values/styles.xml file like that:
<?xml version="1.0" encoding="utf-8"?>
<resources>
<style name="MyTheme" parent="#android:style/Theme.Light">
</style>
</resources>
Then, you just have to apply your theme using the android:theme attribute of the application entity of your AndroidManifest.xml:
<application android:theme="#style/MyTheme">
[...]
</application>