What your primary color does

The primary color carries your buttons, your links and most of the accents. Change it and the whole site follows, which is why it is worth choosing here rather than overriding things later in the CSS editor. If you have a headshot the site is built around, sample a color out of it and the pages hang together without you doing anything else. Very light and very dark primaries both cause trouble, because the design assumes it can put white text on this color.