Changing a Tumblr theme is great for your visitors, but it doesn't change what you see everyday you log in to your account. Regardless of which Tumblr theme you install, your dashboard always remains the same. To change the look of your dashboard, you have to use a dashboard theme, which is performed via your Web browser, not on Tumblr's website. This is a two-step process. First you need to install an app in Chrome or an add-on in Firefox. You can then install a new theme for your dashboard. To find a theme you like, you can use Tumblr to search for one, or browse through a website where Tumblr aficionados upload their dashboard themes.
Finding Themes on Tumblr
Log in to your Tumblr account. Click the "Search Tumblr" field, type "dashboard theme" and press "Enter" on your keyboard. This gives you a list of hundreds, if not thousands, of images of different dashboard themes. Most include a screenshot so you can view how it would look.
Click on a dashboard screenshot that you like. Read the instructions for installing the theme onto your Tumblr dashboard. It should tell you which app or add-on you need to install in order to use the theme and on which site to download the theme. Some may be available with only one Web browser, like Firefox or Chrome. If you don't use that browser, select another dashboard theme.
Download the app or add-on as directed on the Tumblr post and then click the theme's link on the Tumblr post. Click the "Install" button to install the dashboard theme to your Web browser.
Finding Themes For Your Browser
Locate a theme you like for your browser on a website devoted to a single app or plugin. For example, Tumblr dashboard themes on Userstyles.org use the Stylish app on Chrome and the Stylish add-on for Firefox (see Resources for links). Dashboard themes on TotallyLayouts.com use the TotallyLayouts app on the Chrome Web browser.
Click the Chrome app or Firefox add-on link and follow the onscreen instructions to install it to your Web browser.
Click on the theme you want and click the "Install" button to install it to your Web browser.