Conflict Warnings & Resolutions involving css variables

When a property's value has custom variables, the conflict detection seems to throw up its hands and just ignore the issue. When a resolution is specified towards a value that contains a css variable, the resolution value ends up being `inherit` instead.

Start by reproducing conflict detection and resolution with a property value containing a CSS custom variable. Trace the warning and resolution paths to determine why conflicts are ignored and why the resolved value becomes `inherit`; done means conflicts are reported and resolutions preserve the variable-containing value.

Written by the indexing model from the issue text.