Fix autoThemeSwitching when terminal background color hasn't changed but theme is mismatched #20705
Description
What happened?
If autoThemeSwitching is enabled and the terminal background color hasn't changed since startup, but the current theme is incorrectly matched with the background (e.g. light theme on dark background), the theme wouldn't switch automatically because of an early return that bypassed the switching logic.
What did you expect to happen?
The theme should automatically switch to the correct theme if the luminance of the terminal background mismatches the current theme, even if the terminal background color hasn't changed since the previous check.