Squarespace allows 5 text styles: Normal, Heading 1, Heading 2, Heading 3, and Quote. These are configured in Home\Design\Style Editor. If/when you want to use different text styles such as other colors, fonts, underline, etc, then you need to use a code block with HTML and CSS. CSS stand for "custom style sheet". You can enter styles right in the code block, however, there is a place to store these so they can be used all over the site. This can be found in Home/Design/Custom CSS.