Choose a Theme
You can pick a visual theme for your Basecamp account when you click your avatar, then Preferences:
Choosing a theme on the top row adds a splash of color to Basecamp by coloring the Jump Menu, Help icon, and the profile menu shown when you click your avatar.
Selecting the Appearance will change how you see Basecamp - in light or dark mode. You can also choose "Same as OS". This way, if your computer is set to use light mode during the day and night mode when the sun sets, Basecamp can follow that too!
Changes made to your theme and appearance in one account will not sync across your other Basecamp accounts, this should help you differentiate between accounts at a glance.
Themes won't sync between Android and iOS platforms. That means you can set totally different colors on those devices to suit your preference.