A theme is a set of coordinated colors that affect the entire drawing and overwrite any applied fill colors. Quick Styles inherit a theme when it is changed, so they change colors to match the theme but keep the rest of their style. To change themes, simply choose a new one in the Theme gallery on the Home tab.
You can also apply effects, such as bevel, gloss, shadow, or glow, to any selected shapes. Simply click Effects on the Home tab and choose from the menu. Note: any effects, as well as fill and line colors, will be overwritten if you change themes after applying them.
The result is a much more legible map: