You might like to check out:
...which is dedicated to developing OS themes (ie. not Kaleidoscope).
Personally, I agree that most user-designed themes are gaudy and unusable. Mainly because they use a lot of colour, and it is usually incredibly distracting to have unimportant functional elements of your windows competing for your attention with the content of the windows (a real-world example of "style over substance").
Platinum - and older MacOS themes - work so well because they are basically monochrome. With Platinum you can use an accent colour*, but a single colour will not clash with other shades of itself, or with greys...
[*OK it's actually two accent colours, but I always find they work best if they complement each other, eg. light blue, dark blue...]
In fact, even Aqua is mostly monochrome. Yes it has little coloured buttons, and blue accents (just like Platinum has accents), but essentially it's mostly black/grey/white...
less is more and all that =)