These plugins make different CSS themes available for your journal website. See
For simple changes including header colour, link colour, page background colour, foreground (i.e., text) colour, use the Custom Theme plugin.
Once you have made the change, go to Journal Setup Step 5.6 and activate the custom theme.
Save your change and the new theme will appear immediately.
If you change your mind, you can return to Step 5.6 and change the Journal Theme.