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. Writer Bio
