In order to adjust note's appearance right-click elsewhere on note (out of edit area) and choose "Adjust Appearance" item in context menu, or use hotkey defined on "Behavior" page of "Preferences" dialog.
When dialog box appears, select the first tab ("Skins").
Settings on this tab allow you to change note's skin in case you are working in skinnable mode, or change note's body back color and/or note's caption font in case you are working in skinless mode (these options are not accessible for docked notes).
Also on this tab you can set individual transparency level for notes if transparency feature is available.
Button "Standard View" at the bottom of dialog will restore default note's appearance settings for current note: skin for skinnable mode or color scheme for skinless mode.