Bug: Highlighted text invisible with transparent custom themes #4666

Description\nWhen a custom theme sets background to none/transparent and uses a light primary highlight, focused items draw the background color as the foreground so highlighted text becomes invisible (see user screenshot).\n\nOpenCode version\nlocal HEAD (branch fix/custom-theme-highlight-contrast)\n\nSteps to reproduce\n1. Add a custom theme in .opencode/themes/*.json with "background": {"dark": "none", "light": "none"} and a light primary.\n2. Run the TUI (e.g., bun dev) and open the command dialog/autocomplete.\n3. Move selection onto an item; the highlight hides the text. Buttons (dialog actions) show the same issue.\n\nScreenshot and/or share link\nScreenshot from user report (not attached here).\n\nOperating System\nunspecified\n\nTerminal\nunspecified