Modularize margins, paddings, gaps

[eduferfer]

Is your feature request related to a problem? Please describe.
Currently, spacing rules are absolute and can't be modified systematically. This gives little flexibility when theming and makes it hard to maintain styles.

Describe the solution you'd like
Use css properties to define spacing rules and expose them as theming variables.

• gap
• margin
• padding
• top, left, right, bottom