An Option to enable the `system_default_theme` also on the default light and dark theme

**Is your feature request related to a problem?**

Currently `system_default_theme/win10` is only for the System Theme from the Add-on panel.
Can you make an option that the code of [`system_default_theme/win10`](https://github.com/black7375/Firefox-UI-Fix/blob/dev/src/theme/system_default_theme/_win10.scss) also can applied to the default Light and Dark Theme in FireFox?

**The solution I'd like**

An Option to enable the system default theme also on the default light and dark theme

**Screenshots or video**

**Alternatives I've considered**

**Additional context**

Start by reading src/theme/system_default_theme/_win10.scss and inspect how the existing System Theme option is exposed from the Add-on panel. Trace how the default Light and Dark themes are selected, then verify that the requested option applies the Windows 10 styling to both themes when enabled.

Written by the indexing model from the issue text.