I changed my LyX default Roman screen font to the David font.
My text looks like this, with David as font for Roman:
In terms of the font for regular (non-math-mode text), the text has the same font as it had in my LyX before I made any changes to the default fonts.
(for example, if I export the LyX document to PDF), the font change did not change the looks - the fonts in PDF looks the same as they looked before the changes I made to the font in LyX.
But, the problem is that David is now affecting the default math-mode font.
In terms of how the fonts looked before I did the changes, non-math-mode text had a David font (or, at least, it looked the way it looks now), and math-mode text had a different font, whose name I don't know (and don't remember well how it looks, either).
Is it possible to somehow revert back to the default fonts, or does anybody know the name of the default math-mode text font, and it's possible to separate non-math-mode and math-mode texts fonts, and apply the 2 different fonts?