You have several options available from the admin console, you must have an administrator role.
From the configuration option, appearance section.
Also, you can do the editing using CSS at two different points in the configuration and it is always advisable to be managed by people with knowledge of HTML and CSS editing:
- Settings -> Appearance -> Organization Colors -> Custom Header Code -> Complex option and for people with knowledge of HTML + CSS editing.
- Settings -> Awesome -> Custom Styles -> Modifications by Styles -> Even more complex option and also for people with knowledge of HTML + CSS editing.