IMPORTANT:

If you're not using Firefox, you need to remove this part from the code, as well as the final } at the end, otherwise it's just not going to work:

@-moz-document url-prefix('http://www.tumblr.com/') {

You also need to make sure that underneath ‘Applies to’, you click Specify, choose ‘URLs starting with’ and type in http://www.tumblr.com/ (Otherwise, it's going to end up everywhere.)

((Ignore the fact that I haven't updated the screenshots.))