Tumblr lets users edit a blog theme using HTML.
HTML, or HyperText Markup Language, supplies the structure for a Tumblr theme: headings, links, images, navigation, post areas, and other page elements. Tumblr users can modify theme code through the customization interface, while CSS is commonly used alongside HTML to control colors, spacing, fonts, and layout.
A common mix-up is choosing Markdown. Markdown is a lightweight formatting syntax used for writing and formatting text, but it is not the markup language used to build Tumblr’s blog themes. SQL is for working with databases, and Python is a programming language.
Tumblr’s theme system also made customization part of the platform’s identity. Users could create, share, and even sell custom themes, allowing blogs on the same service to look substantially different from one another. The theme editor is separate from the post editor: a user may write a post with rich text or other formatting, but the blog’s overall appearance is controlled by its theme code.