Basic description: Some greater separating one open project from another, especially in tab view.
Reason: When I have many projects open in macOS using the same window with multiple tabs, I often forget which project I had open last, and have to scroll up on the sidebar to see the folder name.
Solutions: Change the colour of the background, tab, or sidebar depending on the path that is open. As background colour might interfere with other styles, using only muted or lightened colours or partially transparent images might also be options.
Implementation: The folder path can be used as a seed to generate a hex value; this way each folder/project path will always have the same colour. Or, the value can be random on each load (would take some re-remembering, but better than each project looking the same). And/or a .sublimerc file could contain special styling.