What's a custon css on tumblr, and what can you put in it?

This is a difficult question to answer and understand unless you have had experience in changing the codes for your tumblr blog. CSS stands for "cascading style sheet" and it is used to change the appearance of your blog. You put the css code in the HTML of your blog. This is the coding of your blog. Different codes can be put in different places to do different things. One css code might add music to your blog or change the background or font of the text. I hope this helps. It is quite confusing. Here is a link if you want to read more. What Is Custom CSS on Tumblr? | eHow