[5.0] Cassiopeia should respect inheritance

[dgrammatiko]

Pull Request for Issue #42260 .

Summary of Changes

• The code for the Colour Theme should respect inheritance and allow overrides, right now it's hardcoded to the parent template

Testing Instructions

• Apply the patch
• Check that the Colour Theme value is respected
• Create a child template test
• Create a file media/templates/site/cassiopeia_test/css/global/colors_standard.css and a file media/templates/site/cassiopeia_test/css/global/colors_standard.min.css with contents:

:root {
  --cassiopeia-color-primary: rebeccapurple;
  --cassiopeia-color-link: rebeccapurple;
  --link-color: rebeccapurple;
  --link-color-rgb: 34, 79, 170;
  --cassiopeia-color-hover: rebeccapurple;
}

Check that when the field Colour Theme has a value standard the template has a purple colour for the links and header

Actual result BEFORE applying this Pull Request

Expected result AFTER applying this Pull Request

Link to documentations

Please select:

• Documentation link for docs.joomla.org:

• No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed

@astridx

[Fedik]

I have tested this item ✅ successfully on 5234af2

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[astridx]

I have tested this item ✅ successfully on 5234af2

Thank you very much @dgammatiko

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[alikon]

RTC

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[HLeithner]

thanks