Hi there,
Thanks for writing in! No there is virtually no limit. Also, it will take tons of CSS to slow a site down unless of course in your CSS you are are calling to large resources like big images all over the place. If you are experiencing performance issues, please check this article.
https://community.theme.co/kb/performance/
You can also run a website speed test to see which areas are taking the most time to load.
http://tools.pingdom.com/fpt/
Hope this helps – thanks!