我使用Samsung Galaxy S9与Android 9。
从Google Drive Android应用程序中的屏幕截图，似乎表明可以更改电池保护程序中的主题：
如何将电池保护程序更改为Android 9（三星Galaxy S9）中的暗模式（三星Galaxy S9）？
I use Samsung Galaxy S9 with Android 9.
Screenshot from the Google Drive Android application, which seems to indicate that it is possible to change the theme in the battery saver:
How can one change the battery saver to dark mode in Android 9 (Samsung Galaxy S9)?
The "Set by Battery Saver" option only means that the app's theme is determined if Battery Saver is enabled or not.
The Battery Saver itself does not have its own theme, but apps can query if the setting is enabled or not for customizing its behavior, including the theme.
On Google Drive (and some other Google apps, and possibly 3rd-party apps), this setting means that if Battery Saver is enabled, then use the theme that saves battery (e.g. dark theme).