`:host-context(.theme)` and `:host(.theme)` are both present in a shadow stylesheet. The shadow tree's host element ITSELF has class `theme`, and so does a flat-tree ancestor several levels up. Considering how `:host-context` walks the flat tree, which rule(s) match and is there a double-match concern?
/* shadow CSS */
:host(.theme) { --x: 1; }
:host-context(.theme) { --x: 2; }
/* host: <div class="theme"> ancestor: <body class="theme"> */