I want to change basicTheme instead of the initial thing "Theme.Black.NoTitleBar.Fullscreen."

Development Platform & Builds Mobile
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For anyone looking for a solution, here’s what I’ve found so far:

When building Android apk, Unreal generates AndroidManifest.xml under Intermediate/Android/arm64. Inside that file, you can check the Android activity declarations. Both SplashActivity and GameActivity is using android:theme="@style/UnrealSplashTheme".

There are two options to modify that:

  1. Update AndroidManifest via UPL
    Got inspired from this post. I added this to the UPL.xml file.
<androidManifestUpdates>
    <removeAttribute tag="activity" name="android:theme"/>
    <addAttribute tag="activity" name="android:theme" value="@style/Theme.AppCompat.NoActionBar"/> 
</androidManifestUpdates>
  1. Override styles.xml file
    During the build process, Unreal will actually copy anything under Build/Android/ to Intermediate/Android/arm64 and override template files. For example, if you set custom App icons, you’ll see it added to Build/Android/res/drawable and then copied over to Intermediate.
    So similarly, you can create Build/Android/res/values/styles.xml and Build/Android/res/values-land/styles.xml to override the styles.xml file. Editing the files in Intermediate folder doesn’t work as they always gets regenerated in the build process! I would just copy files from Intermediate and modify them. This gives you more flexibility to control the style.

That being said, I did encounter another issue after changing theme from @style/Theme.Black.AppCompat.NoActionBar.FullScreen to @style/Theme.AppCompat.NoActionBar. The touch positions are off. They don’t account for the action bar or navigation bar. If anyone has ideas how to fix this, that would be great!