I am having some minor trouble with SeaMonkey 2.49.1 on Linux (Manjaro 17.1.1 KDE). SeaMonkey isn't affected by my Gtk font settings. Instead it uses a really thin and barely legible font. It is the only Gtk (both Gtk2 and Gtk3) program on my system affected by this, and it affects both the official Gtk 3 build, and a custom Gtk 2 build I finished compiling.
SeaMonkey-2.49* only seems to honor the $HOME/.config/gtk-3.0/settings.ini file and not gtk-theme-switch tool. While most other GTK applications seem to honor gtk-theme-switch tool. So find a theme you like using the gtk-theme-switch tool and manually enter the theme name within $HOME/.config/gtk-3.0/settings.ini. Ensure SeaMonkey is using the new (GTK) default theme, and restart SeaMonkey. A good site with other tips concerning the settings.ini file is GTK ArchWiki. As far as font settings, this will likely get you headed into the correct direction...
I just ran into a black box, invisible text. While you are on the page giving you a problem, click on the SeaMonkey-2.49 toolbar, Edit > Preferences > Appearance > Colors. Toggle one or both, "Use system colors" and "Always use the colors and background specified by the web page"/"Use my chosen colors ..." It's a bit glitchy.
I have the following settings:
X "Use system colors"
X "Use my chosen colors ..."
Adwaita-dark theme selected in both settings.ini and the gtk-theme-switch tool:
$HOME/.config/gtk-3.0/settings.ini gtk-theme-name = Adwaita-dark
This really doesn't answer your question specifically, but it may get you headed in good direction, or others may find the information useful.
Additional, gtk-chtheme, edit the font size at the bottom of this tool menu.