Unnecessary dark theme selection in the options

[alexlnkp]

First of all, thanks for making a fork of the youtube-dl-gui! It's a very nice tool.
The options at the moment confuse me, though. Since the app is using GTK, having a checkbox for setting a dark theme is simply unnecessary, since the dark theme is a preference a user can make using a tool like nwg-look.

Without checking any additional checkboxes, my system theme is working correctly:

(Maybe with an exception for the icons, which are rendered as if my theme is set to light, for some reason?)

If I check the "dark theme" checkbox in the options, here's the result:

Which is not at all my system theme, nor is it my system-light theme.

You could make a dropdown menu to select your system GTK theme in the options, and give users ability to select which variation of their system theme they want.
I.e. : a dropdown with 3 options, Default, Light and Dark. Default uses the preferred system theme (as set in GTK options, see [1]), Light forces a light theme and Dark forces a dark theme.

[1]: