Convert to RGB32 instead of RGB24 for displaying on Windows

Avoids a bug in AviSynth v2.6.0 alphas (artifacts on the left-most
column), and it's reported to be faster
Include an alternative dark theme for the script editor

Change the 'reset' button at the bottom for 'light theme' and 'dark
theme' buttons.

The dark theme is still not set.

Related to #29