There isn’t a dedicated Cursor setting to pick a different color just for your messages vs the assistant in Agent/Chat. Those bubbles follow the overall editor / workbench theme, so the main knob you have today is trying another Color Theme (Command Palette → “Color Theme”) and seeing if contrast feels better.
There also isn’t a single “compress all long agent output” control in settings. For very long threads, splitting work across new chats (and using checkpoints when you want safe rollback points) usually keeps history easier to scan: Agent overview.
This comes up a lot — worth upvoting / adding detail on threads like this one and this one so product sees demand.