Use Jira color variables for CSS and provide fallback to own color definitions

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Since the newer versions of Jira and AUI you can use variables to reference colors defined by Jira itself. To support older versions of Jira one should still provide fallback values if no variable definition exists.

• [ ] Use aui specific color variables instead of rgb values in style sheets
  • values can be found on the :root element of every page using chrome developer tools
• [ ] What is the minimum version of Jira suporting those variables?
  • tested with 7.13.18 -> not supported
  • tested with 8.5.11 -> not supported
• [ ] Use fallback colors like this border-color: var(--aui-border, #ff0000);

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This enables dark mode compatibility