There are two ways to open the text style palette:
Highlight the text you'd like to switch to a heading, click on the headings dropdown and choose a heading style. You can click "plain text" to switch back to normal paragraph text.
From here, you can also change text into a button or start a task list.
Click on the More options “…” icon when you have some text selected or go to board options “…” (at the top of the board) > Board styles > Edit text styles to open the text style palette.
Scroll or search for the name of a font in the fonts dropdown
Type a font size in pixels
Set the text color by typing/pasting in the hex code or using the color picker. If you have set your brand colors in Customise on the settings page, you'll see a shortcut to choose these colors just above the color picker.
Learn how to adjust the line height, letter and paragraph spacing.