Only Chrome and Firefox users can do that modification.
Step 1) Install the Stylish:
Firefox : https://addons.mozilla.org/en-US/firefox/addon/stylish/
Step 2) Close your browser and open again;
Step 3) Check this site http://userstyles.org/styles/browse/tumblr.com and search for some theme you like;
Step 4) After you finded the dashboard you want, click in Install with Stylish:
Step 5) Open your Tumblr to see the result.
Note: In Chrome, will appear an "S" in the right upper corner
Click on the "S" when you are in the Tumblr page, and it will appear the Disable the dashboard.
If you install more than one dashboard, it's normal they mix, to solve that problem, it's only click the "S" and Disable all the dashboards that you don't want.
Step 6) Comment here if you get install the dashboard and if you like the result.
Note: To Unistall on Firefox, go to Tools >> Complements >> Will open a new window >> Extensions >> Unistall Stylish.