How to highlight text in a screenshot from safari (or anywhere on the screen) on macOS 14.2

After capturing a screen shot then clicking on it in the lower right corner I plan to highlight some text.

The shot now appears to still be temporarily in Safari. This appears to offer some edit capability but some options are disabled. 

I can...

* With the squiggle icon I can draw a line

* With the Circle box icon I can make a geometry

* With the tex box icon I can add a text box

* !!! With the pencil icon - the pencil icon does nothing ?!?

* With the signature icon I can add a signature

* With the line size icon I can change the size of the line surrounding the text box

* With the outline icon I can change the color of the text box outline

* With the fill icon I can change the color of the text box fill

* With the font icon I can change numerous attributes of the text in the text box

* With the rotate icon I can rotate the entire image

* With the crop icon I can crop the screen shot further

But how the heck do I highlight text or a portion of the screen?

I thought I could make the draw line wide and use that but I cannot change the opacity! the 'Show Colors' does not work here!

I changed course. I then thought I could have highlighting available by opening the screenshot in Preview but the highlight icon is disabled in Preview!

What gives? Why must I go through so much effort to do a thing that is so so simple on an iPad or iPhone?

And how do I accomplish my objective simply while doing a screenshot?