Using custom colors from a theme - bug in Excel 2016?
When you select a custom theme which contains custom colors, you can used those colors initially only with shapes. By doing the following you can use them with cell formatting as well:
1. Select a theme with custom colors
2. Insert a shape
3. Change the fill color with a custom color from the ribbon "home"
4. Select a cell and you can change the color to a custom color as well
If you do not follow steps 2..4 you can not use custom colors. Even more worse if you follow all steps and you want to use the context menu to change e.g. a cell fill it doesn't work either, only from the ribbon home tab.