Popular Web browsers Google Chrome and Mozilla Firefox allow you to run custom scripts that change the way websites render on your computer. Using these features you can create custom backgrounds for your Tumblr login page and dashboard. The changes will take effect only where you have the script installed and won't change what other users see.

User Scripts

Both Chrome and Firefox support the use of custom scripts in different ways. Chrome requires the use of the Stylish extension; Firefox uses the Greasemonkey add-on. Both enable the browsers to run custom scripts that take the HTML code received from the website and add another layer of processing to change it in appearance or functionality. Note that scripts may be helpful, but they also carry a risk since they have access to all the data you see through your browser. Consequently, only install scripts from trusted sources.

Extension and Add-Ons

You can install the Stylish extension through the Google Chrome store (see link in Resources). Once you find the extension, click on the "Free" button to install it and confirm that you want to install the extension. You might need to restart Chrome for the changes to take effect. You can disable or uninstall the Stylish extension through the Google Chrome Extensions settings.

Firefox uses add-ons, which are simply a different name for extensions, and the script add-on for Firefox is called Greasemonkey. Find the add-on (see link in Resources), download it and install it to your Firefox browser. Similarly to Chrome, you can use the Firefox settings to manage your add-ons and determine what they're allowed to do. Greasemonkey also carries the same risks as Stylish since it has the same access to all of your Web browsing.

Styles

There are nearly 20,000 different styles for Tumblr at Userstyles.org (see link in Resources). You can easily find Tumblr themes by searching the word "Tumblr" or by using the browse function. Once you've selected a background theme, you can install the script and refresh your Tumblr page to see the new background. If you want to disable the background you can change it either by installing a different theme or by disabling the add-on or extension that allows the script to run.

Using Styles

Once you've selected a style and installed the script on your browser, every time you browse to Tumblr.com you will see the new visual style instead of the stock appearance of the site. You can change themes as often as you want by installing new style scripts. Additionally, you can apply styles to many other websites using the same process. Userstyles.org has styles for popular sites like Facebook, Google and YouTube. You can even apply styles that will globally affect your browsing experience.