Many Tumblr themes are built using CSS (Cascading Style Sheets), which define how fonts, images and other elements appear. You can either embed the style sheet information in the theme HTML itself, or host it as an external file on Tumblr's servers. The option to upload a style sheet appears when you opt to create your own theme on Tumblr.

In the Tumblr Theme HTML

You can embed a style sheet inside the theme's HTML by choosing the "Customize" button on your Tumblr dashboard settings page for the relevant blog. Click "Edit HTML" to make changes to the theme. Here you can type or paste your CSS guidelines. For style sheets that are more than a few lines in length, hosting the CSS file externally is preferable to embedding it within the HTML.

As an External File

Choose to create a Tumblr theme of your own to access the static file uploader. Select "Manage Your Themes" from the Tumblr theme gallery and then "Submit a New Theme." Click the "Static File Uploader" link to upload your CSS file. Once the process is complete, you will be given a URL to link to in your theme. Note that the Tumblr static file uploader is strictly for theme assets only. You could also use a third-party hosting service such as Dropbox or Google Drive to host the CSS file.