qt: Fix theme loading (#4085)
qt: Fix theme loading (#4085)

* qt: Always use predefined css filenames instead of guessing them when using custom css dir

* qt: Safer getActiveTheme

* qt: Use correct default theme in dashThemeActive
dashpay/dash,thelazier/dash,UdjinM6/dash,thelazier/dash,thelazier/dash,UdjinM6/dash,dashpay/dash,dashpay/dash,dashpay/dash,dashpay/dash,thelazier/dash,UdjinM6/dash,UdjinM6/dash,UdjinM6/dash,thelazier/dash