Refactor theme module as an extension of the css module

[sebnitu]

What changed?

To help reduce complexity and improve usability of the theme module, this PR refactors it to be an extension of the CSS module instead of maintaining it's own custom property map. Internally, that means theme no longer maintains it's own custom properties map, but instead it stores theme properties in css.$_variables. To allow this, css module has been made more flexible by allowing css.get and css.set to more directly access the contents of css.$_variables map.

Deprecated theme API

• theme.get() replaced with css.get()
• theme.log() replaced with css.log()
• theme.$_themes usage replaced with css.$_variables

Additional changes

• Fixed issue with broken watch npm scripts.
• Fixed issue with section component not properly getting root style padding.