[ninjaofawesome]

Although I don't know of any sites that will do this, when I was on the upswing with CSS, I found that making breakable toys was very helpful for me. (For example, I made a very basic site about cat fashion- nothing spectacular design wise, but it taught me a lot about responsive design and let me practice more on my Sass skills as well.)

Perhaps a better tack for you to learn is to find a small business or nonprofit who has a strong brand identity but has no real budget for getting their site made. (I also did this, very helpful.) On the plus side, you'll be helping out someone while you learn, and you don't have to necessarily have great design skills as its already ready already.

Lastly, just my two cents here, getting better at CSS doesn't have to necessarily be 1:1 with design all the time, but it helps to be open to working with designers and having a rudimentary knowledge of design to achieve the look and feeling that they're requesting. So maybe befriend a designer or two and pick their brain a little. You'll probably find that when working with designers in the future that they'll appreciate that you have some knowledge of what they do as well!