You can create new variants for Polaris by creating a new UX Style (sys_ux_style) and setting the type to 'variant'. After saving the record, you will see a m2m related list for UX Theme Styles that you can use to relate your new style back to the Polaris theme. In a variant, you only need to specify the attributes you wish to override - so you don't need to include absolutely everything from the core colors style.
You can also open the existing Dark variant UX Style, insert & stay, and modify the theme as needed. You will still need to relate the style to the Polaris theme using the m2m related list after using insert & stay.
