fix(theme): stop sun/moon overlap and restore dark float contrast

Drive toggle glyphs with html.dark/body theme classes only, drop
inline display styles that lost to !important CSS, and give the
float rail a solid surface + brighter ink in dark mode.

assets/css/frontend/nav/zib-utilities.css

@@ -298,6 +298,7 @@ body.mobile-navbar-show { overflow: hidden; }
   overflow: hidden;
   vertical-align: middle;
   flex-shrink: 0;
+  position: relative;
 }
 .navbar-top .navbar-form .toggle-theme.toggle-radius > i,
 .navbar-top a.toggle-theme.toggle-radius > i {
@@ -308,6 +309,7 @@ body.mobile-navbar-show { overflow: hidden; }
   margin: auto;
   transform: translateY(-50%);
   line-height: 1;
+  /* visibility handled by theme-class rules in nav.css / header-menu-buttons.css */
 }
 /* Menu item title + icon baseline (when menu is assigned) */
 .navbar-top .nav > li > a {