How to grey background text of <code> and <pre> elements on Tumblr?

Question

I would like to avoid having to do this every time I post. So, it should probably be part of my theme template.

Answer 1

You can add custom CSS to your theme without modifying it by going to Customize -> Advanced.

Hence to add a grey background to your <code> you can paste this in the "Add custom CSS" textarea:

pre, code {
 background-color: LightGrey;
}