webterm: improve readability of char under cursor

Make sure it's the opposite of the cursor color.

pkg/interface/webterm/app.tsx

@@ -46,6 +46,7 @@ const makeTheme = (dark: boolean): ITheme => {
     background: bg,
     brightBlack: '#7f7f7f',  // NOTE  slogs
     cursor: fg,
+    cursorAccent: bg,
     selection: fg
   };
 };