There are definitely themes in windows 98. He's not talking about mIRC themes, but your windows theme. I assume that if you haven't touched it you're using default... can you take a screenshot of the issue youre talking about and link to it?
I use windows 98 (original edition, not SE - if that matters), and I didn't saw theme settings, years ago I used XP pro for some months until I digged it and reverted back to 98, and I remember you could select a theme, it asked it at installation time.
If my typed description of the problem is not clear enough, here is a jpg with two screencap areas pasted on it:
