The visible part of PDF highlights are created by SVG elements which are
multiply-blended with the PDF content to enhance legibility of
highlighted text.  The blueish background for focused highlights was
still being applied to the transparent text layer on top of the PDF
which contains the `<hypothesis-highlight>` elements. This impacted the
legibility of focused highlights.

Improve the readability of PDF highlights by instead modifying the fill
color of the SVG element.