Increase highlight opacity

Increase the opacity of highlights from 30% to 40% to improve the
visibility of highlights, especially on very light/white backgrounds. Following
https://github.com/hypothesis/client/pull/2017, we can do this without reducing
contrast of highlighted text in PDFs.

Only the color of "first level" (non-nested) highlights was increased,
as nested highlights already had the higher level of contrast.

src/styles/variables.scss

@@ -150,7 +150,7 @@ $zindex-tooltip: 20;
 // Other Variables
 // -------------------------
 $bucket-bar-width: 22px;
-$highlight-color: rgba(255, 255, 60, 0.3);
+$highlight-color: rgba(255, 255, 60, 0.4);
 $highlight-color-second: rgba(206, 206, 60, 0.4);
 $highlight-color-focus: rgba(156, 230, 255, 0.5);
 $top-bar-height: 40px;