Add support for CSS variables

[cheton]

:root {
  --tonic-ui-theme-colors-gray-100: #151515;
  --tonic-ui-theme-colors-gray-90: #212121;
  --tonic-ui-theme-colors-gray-80: #303030;
  --tonic-ui-theme-colors-gray-70: #424242;
  --tonic-ui-theme-colors-gray-60: #5e5e5e;
  --tonic-ui-theme-colors-gray-50: #8a8a8a;
  --tonic-ui-theme-colors-gray-40: #adadad;
  --tonic-ui-theme-colors-gray-30: #c9c9c9;
  --tonic-ui-theme-colors-gray-20: #e0e0e0;
  --tonic-ui-theme-colors-gray-10: #f2f2f2;
}