option to set theme

### Describe the feature you'd like to request

An option to set the theme (light/dark). Some people have their system on battery saver mode which switches the system theme to dark but still want light UI.

### Describe the solution you'd like

Have an option that sets the theme

### Describe alternatives you've considered

.

Start by locating the UI theme handling and the code that responds to the system light/dark preference. Trace how the extension currently chooses its theme, then define the option's behavior for explicitly selecting light or dark instead of following the system setting. Done means the selected theme remains usable when the system theme changes.

Written by the indexing model from the issue text.