For Chrome and Firefox (I’m not familiar how to install userscripts on other browsers) you have to install Stylish (for chrome)(for firefox).

For Chrome

• click on Stylish icon -> Manage installed styles. -> write new style (on left)
• copy the code from pastebin to code section
• change “applies to everything” click on specify
• change “URL” to “URL starting with” and into box next to it write www.tumblr.com, like this
• 
• If anything won’t show u on your dash, try changing “URLs starting with” to “URLs on the domain”
• On the left, give a name to your theme and make a tick into enable box
• click save and voilà, you have a Tumblr dashboard theme

For Firefox

• Click on Stylish Icon (If you don’t have it, click on menu bar -> customize -> find the button with “S” and drag it to menu bar)
• Click “Write a new style” -> “Empty style”
• Copy-paste the code
• Give your theme a name
• Here you go, Dashboard theme for Firefox