Differentiate site default and browser default theme

[SleeplessOne1917]

Requirements

• This is a bug report, and if not, please post to https://lemmy.ml/c/lemmy_support instead.
• Please check to see if this issue already exists.
• It's a single bug. Do not report multiple bugs in one issue.
• It's a frontend issue, not a backend issue; Otherwise please create an issue on the backend repo instead.

Summary

Currently, if a user sets their theme to "Browser default" and the site has a theme set (e.g. vaporwave), the user's view will use the vaporwave theme instead of the browser default. The name "Browser default" suggests it will let browser settings handle the theme, but a set site theme will override this.

Steps to Reproduce

1. (As an admin) set the site theme to a non-default theme
2. Set you user theme to browser default
3. Notice that the site theme will be used regardless of whether the browser is set to use light or dark mode.

Technical Details

n/a

Lemmy Instance Version

0.19.3

Lemmy Instance URL

No response