Use CSS variables instead of color theme Velocity variables in the Solr search stylesheet

The stylesheet of the Solr search page (the StyleSheetExtension object of Main.SolrSearch) is parsed with Velocity only to load colorThemeInit.vm and read 5 color theme variables: textSecondaryColor, highlightColor, titleColor, textColor and borderColor. The CSS variables introduced by XWIKI-22667 already expose the same values, so the stylesheet can use them instead and no longer needs Velocity parsing.

The search page looks the same, and its stylesheet no longer has to be evaluated by Velocity. The only exception is with an old-style color theme that customizes textSecondaryColor: the muted texts of the search page then use the default muted text color of the skin, since that color theme variable is only mapped to --text-muted for the LESS-based color themes.