Request/food for thought...

[paaguti]

Include selection background as part of the customisable colours.
I've created a theme with bg1 #dfdfef and have found that #deefef is a nice choice for the selection background, Much more that any of the bg2, bg3 or bg4.
This new selectionbg field can be set initially to bg3 or bg4 (which can work with some themes)
The point is to make it user configurable

Thanks

[paaguti]

I have continued this line of thought a bit more. This is a theme I have created with the current GUI:

This is the theme after a few tweaks, mainly the selection colour definition I suggest:

There are a couple more things like a darker fringe for light themes (lighter for dark themes) and an adjustment of the tab-line to this change.

This is the diff between the original and the tweaked theme

theme-creator.zip

Hope to make the (Emacs) themes a bit better ;-)