First of all we are really glad to help webdevelopers in making their sites work good in Opera Mini.
In general CSS support is really similar to what you have in Opera 12.16/12.18 so you can test your site first in old Opera desktop browser as it's probably much faster way to develop. There are some exceptions and differences:
- in most cases possible font-sizes are limited to some subset, I described it in detail in another topic, notably this subset differs between devices,
- line-height is disabled at the moment, though we consider enabling it in the future, so please don't make assumptions on this,
- padding should work fine,
- elements with position: fixed are rendered as they were with position: absolute - they don't stick in viewport when you scroll,
- no CSS transitions, limited transforms support,
- broken complex flexbox-based layouts, simple things may work,
- some other minor things.
Regarding tests you can perform at css3test or w3c sites: they show what the engine supports, but a lot of things are changed on transformation to OBML.
If you have any specific issues you can always post your page or example problem here and I can assist on possible solutions/workarounds.