Dark theme improvements

[manfromarce]

• There are few issues in Windows file / folder pickers when using the dark theme that doesn't occur in other apps. For example, you can open these dialogs from Settings > Editor > "..." and notice some flickering and odd behaviors, and the background color is different from expected: This is how the standard dialog look when opened from Windows Notepad (and also flickering doesn't occur):
• Windows message boxes that are shown when e.g. removing a file from an archive have an inconsistent theming:
• The "Go to parent folder" button near the address bar has a white background when using dark theme
• It would be nice to have a setting to use light, dark or system theme rather than always following system theme