How do I change the look of Linux Mint?

To access the desktop appearance settings, go to Menu > Preferences > Appearance or Menu > Control Center > Personal > Appearance. The window that opens shows three basic tabs which are Themes, Backgrounds and Fonts.

Which is faster Ubuntu or Mint?

Mint may seem a little quicker in use day-to-day, but on older hardware, it will definitely feel faster, whereas Ubuntu appears to run slower the older the machine gets. Mint gets faster still when running MATE, as does Ubuntu.