fix(style): replace bg colors with css variables

[radupasparuga]

Implemented changes

In this PR we remove some hard coded background colors and replace them with css variables so that elements are easier to customize

Screenshots/Videos

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[silvester-pari]

Thank you for looking into this! I had a quick look and found some small things:

• good idea to use the var, but maybe we should set it to transparent as fallback: --background-color: var(--eox-background-color, transparent);. Currently the fallback is white, and then we'd need to manually set it to transparent. Affects: drawtools (see editor story), itemfilter, layercontrol
• some elements are missing. Affects: itemfilter, jsonform (.CodeMirror)
  (see screenshots)

[silvester-pari]

🎉