In order to do that we must change the text layer opacity to 1 but
it has several implications:
- the selection color must have an alpha component,
- the background color of the span used for highlighted words
must have an alpha component either, but now the opacity is 1
we can use some backdrop-filters in HCM making the highlighted
words more visible.
- fix a regression caused by #17196: the css variable --hcm-highlight-filter
has to live under the #viewer element because in HCM it's overwritten
by js at this level, hence links annotations for example didn't
have the right colors when hovered.
83c78dbfa8