Here you can find color themes for your power bi report. The advantage of this theme generator is that it generates 100 colors at once. This because of larger reports that use alot of charts and diagrams which uses alot of colors to display measurements, runs eventually out of colors. When this heppends the standard colors kicks in and the report theme will not be as good looking as it should.
In Power Bi Desktop, you find it under the "View" tab, and under the category "Themes". Click on the "arrow down" and fianlly click on "browse for theme".