CSS: Simplify the header bar to avoid absolute positioning.

Since we now rely on the natural flow instead of absolute positions for
the sidebar and main contents, we have the absolute certainty that there
won't be any gap even if the user forces a bigger font. Also changes
won't have to be reflected at other places anymore.

It causes no aesthetical change at all.
CSS: Fix the gradient of the header bar on mobile devices

Previously, when the width was small enough, the gradient would
collapse into a bi-coloured white/brown area with a hard transition.

The new gradient looks nicer while being close to the old look for
medium-sized viewports.
CSS: Give a more modern look to the search bar.

The previous look with the bevelled input looked so '95-ish.
CSS: Fix linear-gradient syntax for non-vendor-prefixed version.