While we have 10 base colors you can define in the color scheme menu, we actually generate 22 more colors from those that are used throughout Discourse (you can see all the colors here, this page was generated by our Styleguide plugin).
In this case specifically, your primary
color #f8f8ff
is slightly blue, and it’s being darkened to create suitable contrast so it stands out against secondary
(the site’s background) while primary
colored text is still also readable on top of it.
You can experiment a little to see the effects. If you change primary
slightly to #eff1f4, for example you get
or if you remove the blue from your primary
and make it #f8f8f8
you get
If you want to dictate specific colors for every element, custom CSS is the way to go.