You can embed CSS definitions in the general style sheet of your website. The cookie dialog/banner will then automatically adopt these. Please also see our guide for more details on styling: Styling the Elemental banner.
Do note that, if the Cookiebot standard templates change in future, you may need to adjust your styling accordingly. While we cannot make any guarantees about the future, we have no intentions of making changes to our use of HTML, IDs or anything else that could break your styling. We are very aware of this and know that it would affect the majority of the Cookiebot users since most style their banners using the built-in styling options and additional CSS (only a minority build their own banners from scratch using the ‘custom’ template).