F&O doesn't allow the personalization of the colors in the themes. There are only 15 options available that you can choose from, and there's no way to change them. If you want to set a Dark Mode or set your company colors, this is not currently possible.

It would be great having the option that allows the themes' extension, or a way to customize the cascading style sheet, so in this way each client can configure a set of color that matches the personal preferences or the client's brand guidelines.