I think the php referring back to colours in config.php has actually made the CSS more complicated because it makes it too easy to repeat the same value in many places. In CSS you should only have to define a colour once per 'design decision'. I'm not sure it's obvious what I mean by that, I'll try to demonstrate:
Currently (as of the last time I looked at styles.php) each 'thing' defines it's styles separately e.g.
.thing1 {
color: pink;
}
.thing2 {
color: pink;
}
If, and only if, these are both pink for the same reason, and it would make sense to change one when you change the other then this is more appropriate:
.thing1, .thing2 {
color: pink;
}
Obviously this expands so that every item that is pink (for the same reason) is set together in a single style declaration. Therefore there is a single place to change each decision about color, font, border etc. I went through and redid one of the standard themes in this way. I'll see if I can find the file and you can compare them.