Fresh installs and new profiles now default to the Fusion (dark) theme
instead of the platform-native theme. The old "Default" theme is
renamed "System" to better describe its purpose — using the OS-native
Qt style (windowsvista, macOS, etc.).
Existing users who had "Default" selected are automatically migrated
to "System" so they keep their platform-native styling. Users with
an empty or invalid theme name now fall back to Fusion.
* add button to open themes location to settings
botton creates directory if it doesn't exist yet
themes path is no longer hardcoded but included in settings
themes now default to None the default theme is no longer required
themes set to None will not look for empty directories anymore
this is backwards compatible
users with a nonexistant theme (Default) set will get the new None theme
* remove default theme from install instructions
