Whenever I have seen that sort of problem, it's been due to an add-on. Does this problem persist if you run Thunderbird in its Safe Mode?

No, but in safe mode the icons aren't as big or as colorful or as recognizable. What happened? I'm using something apparently that returns the TB2 icons, and have been for ages. I actually didn't realize I had an add on, it's been so long. Is there something else, another add on, that might be better?

It saddens me that Thunderbird's default icon set is all grey, yet most of my addons that bring in additional buttons use colours. The inconsistency really bugs me. I accept that users with colour impaired vision may not be interested in coloured icons, but for the vast majority of us who have normal vision, it makes sense (to me) to use colour deliberately for its ability to help differentiate between controls.

My own favourite themes have not been upgraded to work correctly in TB45 and for the moment I'm working with one (Phoenity Shredder) that gives me colours and a reasonable use of space but messes up with one specific addon. Fortunately, this addon also works through a menu item so I can live without its broken toolbar icon.

Some other themes that give me colours (Littlebird, Tinybird) also impose a cramped and cluttered layout and I simply can't be comfortable with these. And there is yet another class of themes which look nice, but omit certain critical icons or buttons. "TB2 reloaded" is one of these.

I should also point out that as a Linux user, I see several attractive themes marked as "not available for your platform". If you use Windows, you may have a wider choice than me.