The issue comes from the use of badge-secondary and span.highlight
the text is white and the highlight is yellow, so no contrast.

Can be solved to get

By modifying the css to

span.highlight {
background-color: #ffffcc;
font-weight: bold;
padding: 1px 4px;
color: #343a40; // color added
}

But also, obviously the .badge text is much too small and its padding is not enough.
By modifying the .badge css to use

padding: 0.45em 0.8em;
font-size: 85%;

we would get

What do you think?

Will make PR later.

@infograf768 did you forget?

I was concentrated on the damned svg...
Will do tomorrow.

NP. just shout if not and I can do it

Closing as we have a patch.
#27914