Should you know CSS?
Yes, absolutely. CSS stands for Cascading Style Sheets and it’s CSS that flavors the visual experience of your website. A good portion of the time this type of thing will be handled for you by the CMS and its developers; however, there will be times when you run into a problem that is solvable if you have an inkling about CSS and completely unsolvable if you do not.
There are many free options to getting a basic introduction to CSS online. I would highly recommend Lynda.com—for a small monthly fee you can get access to a wealth of great tech videos. This will give you a few options on what to learn about CSS, but start with the most basic first.
Even just getting down some of the basic structure can help you articulate issues that come up with the website—so even if you don’t fix the problem, at least you’ll be able to relay the issue to someone who can. It’ll go quicker and everyone will appreciate the clarity.