It might take a bit of experimentation, but it does appear that you should able to make that them by editing the existing theme variables. You can find them in _sass/theme with _default.scss being the default theme that's normally applied and _dark.scss being the dark theme. Adding the changes a new theme and then setting the site_theme setting in _config.yml to the name of the theme (i.e., the filename without the .scss extension) should work although you may need to disable to the toggle for light / dark mode.