Question #1
Long story short, I accidentally deleted my default Window Skin in the RTP folder, reinstalled another copy of the RTP, copied the missing default skin, pasted it into the RTP System folder, then uninstalled that copy of the RTP.
Weird thing is, now the default Window is taking on a GREEN colour instead of the usual blue. How did that happen? I tried to search for solutions online but couldn't seem to find it.

Question #2 - I tried to test out new Window Skins (this time by importing it!), following the advice through this video.
The weird thing is, the colours seem off after I import the new Window Skin. It doesn't match at all with the Window Skin PNG. I thought I had to use this tweak, but the colours still seem different. Is there something I'm doing wrong? Thank you in advance for anyone helping!