My gratitude for your response, Tony,
With the wikitext markup, I was under the impression it was for the tiddler text body while everything else would be handled by CSS styling and why I was thinking the markup wouldn't mix well. I am doing like you suggest and going with it. Basically that's how I'm learning TW, taking what I see all other previous TW'ers done, applying and often tinkering with it. I do like TW's preview panel which often shows the results as I try them.
I know I could use styling to do the same, but my Windows and browser overrides all styling so unfortunately I have no idea what TW with its basic themes looks like. That also means I cannot safely alter the visual theme since my TW's with be provided to others with default themes. That was why I was asking elsewhere about an alternate to highlighting something by text or text background color-- such doesn't work with my system setup.
What does TW with default theme do to indicate the current selected sidebar tab, I cannot tell. With vertical tabs, one side of the tab edge disappears.
Which makes your suggestion about icons in captions, very interesting -- I could differentiate content tabs from the stock Open, Recent, Tools, More tabs. Your menu's TW will keep me busy even more. As we often joke with our cats-- Stuff! Things!! Pounce!1!
Even tiddlywiki itself places icon transclusions in some tiddlers caption field,