implement CSS custom properties

[moebiusmania]

(aka: variables)
to add visual customization https://developer.mozilla.org/en-US/docs/Web/CSS/Using_CSS_variables .

also https://lit-element.polymer-project.org/guide/styles#css-properties

to limit the scope of this issue, via CSS custom properties an end user should be able to implement basic theming/skinning like:

• main color (the titual amber)
• states (implemeted in button, banner & modal)
• priorities
• backgrounds if needed