Works OK but imho the default bootstrap $mark-bg: #fcf8e3 !default; is too weak.
we have

mark, .mark {
    padding: 0.2em;
    background-color: #fcf8e3;
}

gives

I suggest overriding it with a stronger hue in .../templates/cassiopeia/scss/tools/variables/_variables.scss

for example: #fbeea8

which would give

It looks good,
but I have a couple notes :)

Now something more complicated ?

1) We do not need layout joomla.highlight.highlight.

You should not define ALL possible options in the layout.
The JS script should have a default options, and merge with User options, in this way it will be more efficient.
So, example, if your code have 'iframesTimeout'=> 5000, 'class' => 'js-highlight' then this should be in JS default, not PHP.
Then in PHP set only modified options:

$doc->addScriptOptions('highlight', [
 'highLight' => $terms,
])

Developer should set them only if he want diferent class, timeout:

$doc->addScriptOptions('highlight', [
 'highLight' => $terms,
 'class' => 'foo-bar',
])

In this way we save a loot of KB, and we do not need joomla.highlight.highlight.

2) For this script it better to allow to have a couple highlight on the page.

In addScriptOptions set array of options, instead of just options,
in this way we can have a couple highlight, for different terms:

$doc->addScriptOptions('highlight', [[ 'highLight' => $terms]], false);
$doc->addScriptOptions('highlight', [[ 'highLight' => $terms2]], false);

The JS could do some caching/checking to avoid the terms duplication, so we safe if someone have multiple call of same terms.

Now something more complicated

Should be ok now, fwiw I was copying fixing the old PR here without paying too much attention on my previous assumptions

@infograf768 as this PR is just replacing the highlighting script can you make a PR with the colour change once this is merged?

Edit: nevermind I added it here

I still would drop from php 'accuracy' => 'partially', 'diacritics' => true,
But it already better than before ?

Later I will try to test

okay, I have tested, and it does not work ?
well, it partly work, only for ?highlight=WyJjcmVhdGUiLCJhZG1pbiJd
but not work for search result view

I have tested this item ✅ successfully on 86aaaf7

I have tested this item ✅ successfully on 86aaaf7

Created a highlight string to match content I had on my J4 installation and highlighting appeared as per expected result.

RTC