People vary in their ability to process colors, contrasts, and the like. On the Color Themes page you can select a combination of background and text/tint colors from the rolling picker that will best fit a particular user's profile. That match will improve their access to the information on the screen.
Rolling the picker will change the theme in the navigation bar, providing a preview of the change. The theme of the picker page itself will not change, because the overlap of colors (i.e., existing and new themes) would be confusing.
If you turn on the toggle switch at the bottom of the Color Themes page, that label will change its theme as well to show you that you are saving the selection as the new default (i.e., it will no longer be blue text on a gold background).
In order for this new default to actually be used by the program the next time it starts up, you need to go to Settings and turn on "Use Default Theme for Startup"; otherwise, the applied theme will remain blue text on a gold background.