Global CSS variables extension

[untael]

We should have following global CSS vars:

• --va-box-shadow
• --va-font-weight
• --va-border-radius

PS: should be extended

[m0ksem]

According to #720 we also need global CSS for border-width.

[asvae]

@m0ksem Maybe we can introduce something like a customization layers/pyramid? As a recommended way to customize vuestic.

config (services config)
  config (default props)
    global-css-variables
      local-css-variables
        bem overrides

[asvae]

Needs more planning. Ideally in context of this issue we want to have a list of all possible global variables.

Then mark the ones we're going to implement.