unfortunately there is no variable support at the moment because manual editing isn't expected to be the best way of customization considering all the different combinations of elements in the UI.
manually editing the theme files is of course always possible. however this is not the recommended way as theme files ship in the default installation, usually get updated rather often and at times the scopes (the categories starting with ".") change at times.
the way that is supposed to be the best over time is to use the builtin UI customization. this tool will only write overwritten theme values to the ui.config file which will be preserved when unzipping updates and will even exist across theme changes. the current state can then also be saved to a finished theme file using the "save as" text field in the menu.
however at the moment this tool is rather "developer quality". in the current version there is a element picker bound to "ctrl+shift+alt x" by default which can be activated and then locked with the same shortcut. the overlay on the bottom can then be used to customize individual elements one by one.
in the next update this will be replace by a full customization overlay which also has a sidebar to allow for customization based on the scope of elements and to change all matching color values at once (kinda like a bad imitation of variables, but more like search and replace). this new overlay also avoids the problem of overlapping the actual ui and will be available from the menu instead of a shortcut. the element picker will still be available from this new overlay instead of a shortcut.
the next update will also replace all existing themes except "Dark" which was always the only actually usable color scheme. There will be new themes in gray and blue flavor that will have less areas with unreadable text and should be more in line with colors that someone actually would want to use on a day to day basis.