CSS variables avoid having to repeat code for each alert type.
Also remove the need to add "tip" to the theme-colors (no need for "tip"
styled buttons, badges etc, especially as tips use the primary color) by
styling `.alert-tip` with customized copy of BS alert mechanism.
In order to be able to modify the icons with a CSS variable,
the `content` attribute has been removed from the `o-inline-icon` mixin.
Code where this mixin is called in other places has been adapted.
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