Inside of VRChat, open up the main menu and navigate to the VRChat Plus tab, where your other custom UI themes and various VRC+ settings are stored.

Choose the UI Themes section on the left side, click the plus icon to create a new theme, and name it whatever you want.

On the screen that pops up, press the Import button.
A keyboard will pop up. Paste in the code (It'll look something like '#F4EBD0, #0d3537...') that you got from this site, and you're done!