Most of this screenpart is controlled by portalcontent.css.Active Tab:

Appearance: portalcontent.css => .TabHiFont
Inactive Tab:
Appearance: portalcontent.css => a.TabDimFontSpace between tabs:
Appearance: portalcontent.css => .TabSepThickness of the bar below the tabs:
Appearance: portalcontent.css => .TabLineTableTab Background image's
Appearance: .TabHiCell, .TabHiSep, .TabDimCell, .TabDimSepMinibuttons:
Appearance: portalcontent.css => .minibuttonOnDashboard Sections:


Of course this posting doesn't include all possible CSS settings but it gives you a good starting point.
Till Next Time
This article was original written for the Ciber Knowledge Blog: