In that case, highlighting headers can be useful, but at this time it's difficult to highlight phrases in 'normal' text blocks because there's nothing specific to target.

You could add css to look for phrases in a block of text but at best the css would apply to the entire block, not the target phrase(s).

css for highlighting all headers is easy now that every header gains a css id for use with making a TOC.

css in use here:

.dark [id^='md-']:not(#md-section){
background-color: #006600;
border-style: solid;
border-width: 1px;
border-color: #000000;
padding-left: 4px;
}

[id^='md-']:not(#md-section){
background-color: #00CC66;
border-style: solid;
border-width: 1px;
border-color: #000000;
padding-left: 4px;
}

Basically what this is doing, is targeting any element with an id that starts with 'md-', but not the exact id md-section(because this is the id associated with blank h1 headers - which is how you make a light dotted line separator). This css adds a background and border, the background color is different for normal vs dark view.

As previously mentioned, you can apply some css to empty urls and then use that to 'highlight' select text within a block, however, this would be limited in that you cannot then highlight a working url; you would at best be able to highlight text on either side of a working url.

You'll be able to see that I've added css to the TOC links here, the css for which is as follows:

[href*='#md-']{
background-color: #222222;
padding: 2px;
}

What this is doing, is targeting elements with an href containing '#md'-. I opted with contains rather than starts with so that TOC links that contain the full url of the submission(eg when posted in a comment) also receive the css.

@srayzie