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).
- Dark: always use the dark theme
- Light: always use the light theme
- Set by Battery Saver:
- If Battery Saver is off: use the light theme
- If Battery Saver is on: use the dark theme