Plain CSS skin customizations with custom.css

Custom skins can now add their own styling in plain CSS by overriding the new custom.css file of the Flamingo skin. It is appended at the end of the skin's main stylesheet, after all its LESS styles, and can use the CSS properties of the skin, such as var(--brand-primary). The less/custom.less file is deprecated in its favor, but is still imported so existing customizations keep working. Learn more about it.