thanks for your answer!
well what I'm showing is definitely not a typo, it's what the theme generated, for sure

The theme is an MTS 1.1 theme. Could very well be that is was written for mIRC 5...
anyway, the thing is, I first tried to use the /view/font dialog to set the font to what I wanted, but I can't switch it to what I want it to be. The smallest possible Tahoma I can select is 8, and it is slightly too big. mIRC then sets this in the ini file:
[fonts]
f#testfonts=Tahoma,411,0,2,0,8
My first guess was to change the 8 to 7. But then it's too small. I don't know, I guess what I want is 7.5?

but 7.5 seems like an illegal value to set in the ini file because the font size does not change if I do that.
Which is why I then tried to apply the older setting of
[fonts]
f#testfonts2=Tahoma,410,1
and that worked for all windows except for the text input field, which why I wanted to know if there is a variable name for it... but I guess there isn't, and the field should "usually" take the same font as the window?
interestingly enough, if I use 411,1 then the font is too big again...

are the different values/settings documented anywhere? I couldn't find them in the mIRC help or by googling...