Jonny, thanks for the clarification. It would have been easier for us if you had mentioned your tweaking right at the beginning.
OK, what I can tell is that when you set browser.display.use_focus_colors to true, then this will also affect dropdown lists. I think that's correct since the focus colors are meant to help you finding the currently active input element.
However, when you set browser.display.use_document_colors to false, the effect on buttons and dropdown lists doesn't make sense to me. I think that's a Mozilla bug (SeaMonkey behaves the same way).
As an alternative, I can offer you a couple of CSS rules. Open Edit > Configuration > User-Defined StyleSheet and paste the following:
/*
* disable background images
*/
html * { background-image: none !important; }
/*
* use system colors instead of document colors
*/
body * { color: inherit !important; background-color: transparent !important; border-color: inherit !important; }
html > *, input, select, textarea { color: WindowText !important; background-color: Window !important; }
button, input[type="button"], input[type="reset"], input[type="submit"] { color: ButtonText !important; background-color: ButtonFace !important; }
/*
* set link colors
*/
a:link { color: blue !important; border-color: blue !important; }
a:visited { color: purple !important; border-color: purple !important; }
a:active { color: red !important; border-color: red !important; }
a:hover { color: red !important; border-color: red !important; }
You have to restart km to make changes take effect.
These rules have pretty the same effect as setting browser.display.use_document_colors to false (you have to set it to true again), but don't have that strange effect on buttons and dropdown lists. You may change the link colors to your liking. You can also combine these rules with focus colors.