Allow selecting base themes as well

Allow selecting themes from the base folder `C:\Windows\Resources\Themes` as well. Currently, the program does include default themes in its listing, but any additional themes added to that folder will not be recognized. The only workaround is to build a new custom theme using the new theme, then save it to `C:\Users\[User]\AppData\Local\Microsoft\Windows\Themes` for it to be selectable in the program.

Start by tracing the theme-discovery code that builds the program's theme listing, comparing its handling of C:\Windows\Resources\Themes with the existing custom-theme folder. Done means additional themes in the base folder appear as selectable entries while existing default and custom theme selection continues to work.

Written by the indexing model from the issue text.