Solid color wallpaper support

• Adds new color slot wallpaper, defaults to a slightly darker bg (panel background color)
• Redmond XX and XX-SE updated to feature my favorite color as wallpaper.
• Redmond XXI updated to feature wallpaper color same as Win2000/Me default color (#396ba5)
• Small fix for navbar not properly applying topBarLink color
• Small refactor to how background image is applied
• Made it so that wallpaper is horizontally centered and top-aligned (alongside navbar)
• Added an option to hide instance-defined wallpaper if it's present and if user doesn't have their own wallpaper set up.

src/components/desktop_nav/desktop_nav.scss

@@ -5,6 +5,10 @@
   width: 100%;
   position: fixed;
 
+  a {
+    color: var(--topBarLink, $fallback--link);
+  }
+
   .inner-nav {
     display: grid;
     grid-template-rows: 50px;