Small header design fixes (playfulprogramming#1240)

Fixed clipping of the CSS icon in the banner by making the header
transparent until a scroll is detected. Removed the transition duration
as a 1px divider isn't gonna make or break anything.

Fixed doubled theme buttons on desktop thanks to @fennifith

Author: edpratti

src/views/base/navigation/header.module.scss

@@ -25,13 +25,12 @@ header {
 	border-bottom: 1px solid transparent;
 	&[data-sticky="pinned"] {
 		border-bottom: 1px solid var(--background_disabled);
+		background-color: var(--background_primary);
 	}
 
 	top: -0.1px;
 	position: sticky;
 	z-index: 2;
-	background-color: var(--background_primary);
-	@include transition(background-color);
 }
 
 .header {
@@ -153,7 +152,7 @@ header {
 			display: flex;
 		}
 
-		.themeToggleLarge + .themeToggleSmall {
+		.themeToggleLarge ~ .themeToggleSmall {
 			display: none;
 		}
 	}