At the bottom there is a box where you can enter style overrides. In that box you can enter CSS style rules as part of the GLOBAL_HEAD element of your journal, which affects all of your journal pages (recent, friends, calendar, day).
Note that if you are using more than one GLOBAL_HEAD override (such as a font override and a background override together), that the overrides will need to be merged together (https://www.insanejournal.com/community/howto/1020.html).
(Remember that font names may be case-sensitive, and must be spelled correctly, with any spaces in the correct places, in order to work properly):