Add a section to define variables

[RamonEspinosa]

The theme editor works great, but maintaining a theme can become hard for people who want more control over the components.
Imagine the following case:

1. A designer updates all color properties to match a color palette.
2. The new theme is reviewed and they want darker colors. Hence a new color palette is chosen.
3. The designer has to go through the whole interface and update every single color property 😭

I propose to add a section to define custom variables. When defined, variables will be then converted to CSS custom properties and could be used to define the value of a property in the theme.