To put it shortly, the current selection colour makes some styles hard to read. Instead of modifying the - otherwise well done - style sheet, I wanted to modify the selection colour.
My first approach was changing the **selection** settings in filetypes.common, but this did not have any effect.
After some researching, I found a hint that the selection settings in the theme file take precedence (which makes sense to me), so I put them there as well. I made shure (for testing only) that the selection settings in the theme file are very different from the settings in filetypes.common, so that I can see which ones will effectively apply. However it seems that both are ignored.