What would be the proper way to implement a theme-red variant?

Minor edit on issue title.

One ugly way out (a hack) is to set the theme to e.g. Black and then copy the CSS from the mmenu.js library and change it to this:

.mm-menu_theme-black{
  --mm-color-border:rgba(0,0,0,0.25 );
  --mm-color-button:rgba(0,0,0,0.4 );
  --mm-color-text:rgba(0,0,0,0.75 );
  --mm-color-text-dimmed:rgba(0,0,0,0.4 );
  --mm-color-background:#ff0000;
  --mm-color-background-highlight:rgba(0,0,0,0.2 );
  --mm-color-background-emphasis:rgba(0,0,0,0.15 );
  --mm-shadow:none
}