SVG support on Windows is added in #3082. Then GTK and themes can use librsvg and so display SVG images. This is probably useful in general. @techee thanks for the pointer.
https://github.com/geany/www.geany.org/pull/40 adds a FAQ item explaining how to change the GTK theme on different platforms. The macOS part yet needs to be written. For the windows part, I added the themes mentioned here as suggestions, hoping this will help users to get a start. I'd say further discussions on this topic could go into the PR to not clutter this issue too much.
About bundling a theme on Windows: if we do, I don't mind much which one. I just don't know yet (but also didn't try yet) how to set the GTK default theme without overriding a maybe existing user configuration or whether we install `settings.ini` into Geany's installation directory but then it's harder for the user to override the theme if she doesn't like our choice.
@alexnerd123 about the low contrast in my screenshot: can you try the theme on your system? I only have a Windows 7 VM and I never will use any newer Windows version, so my screenshots won't get better.