Speed: Avoid CSS @import

[delisma]

Describe the bug
Avoid using @import in CSS files as this prevents parallel loading of CSS. Inside a CSS file, @import can be used to include the contents of another CSS file by specifying a URL. This can be convenient but impacts download times because browsers can only start fetching the imported URL after the CSS file containing the @import has been fetched.

To Reproduce
theme.min.css
wet-boew.github.io/themes-dist/GCWeb/GCWeb/css/theme.min.css (2 CSS imports)

Expected behavior
To get a page to load CSS files in parallel, we should instead add a <link rel="stylesheet" href="..."> tag for each of the CSS files we need to load to the HTML code of the page.