Using custom CSS, you can add a colored background to your posts without editing your posts in other ways and without changing the background of the rest of your blog's body.

Changing the text style in the HTML is possible thanks to Cascading Style Sheets (CSS), the stylesheet language used to describe the presentation of a document in HTML.

One way to add a background photo to any custom Tumblr theme is to embed a simple CSS tag at the right location in the theme's HTML code.