Refactor default theme for better maintainability, bring CSS variables and stylelint

[ronilaukkarinen]

Hello! Finally got a project where I had to use this and could not go with the current structure because of maintainability. Related to #60 decided to go and do the work. This PR will maintain the current default theme completely refactored carefully. I have tested this with the demo and it seems to work fine.

This PR will:

• Get rid of unnecessary and overly complicated nesting
• Tries to prevent too much specificity, although some parts are forced to be ignored in sake of functionality, may be further simplified though if time
• Get rid of unnecessary overrides
• Replace unspecified and unclear darken() colors with real variables
• Prefer specificity in favor of !important rule to maintain continuity
• Remove unnecessary & and > that are not needed at all
• Fix duplicate selector issues if any
• Disable overcomplicated calculations in font sizes, prefer CSS variables instead

After this making a new custom theme will be easy. You just need to replace the colors in variables with your own and style other things according to preferences.

[somewebmedia]

I'll check this as soon as I have a chance for it :)

[thagxt]

Any plans of merging this in master? It would be pretty useful to have CSS vars for easier customization of the drawer.