Depending on the client, you just type in the new font.
To view the content, you need to sign in or register
Above for example (QuantumFusion's 1873) you go to the very bottom of the exe and replace "COMIC SANS MS" with whatever font you want, nulling out the excess bytes. A lot of exe's dont have it at the very bottom, but changing the font is the same.
This can be done through Olly as well. Find CreateFontA and search for references.