For both custom and default themes
Start, Taskbar and Action Center grayed out
If you are using a light theme on your computer, you will notice that the Start, taskbar, and action center option in the Color settings menu is grayed out. This means that you will not be able to touch and edit these in your settings. The reason why this option is disabled is because if Windows allows you to change the accent color of these elements in a light theme, these elements may not be visible on your screen properly.
However, if you need to work with the option and you want it to be enabled, there are a couple of ways to do it. Basically, you can just go into the Settings app and enable an option, and it will then activate the option for you.
You can enable the option for both default and custom themes, but the steps differ slightly for each theme type.