tl;dr: Two patches: One recolors the green emerald text box to the ruby/sapphire blue colors, the other changes the Emerald font to the Ruby/Sapphire font.
Why, you ask? Well, Since it doesn't make sense to hack Ruby anymore, for nostalgia reasons or other, you can use this patch to make Emerald still feel like ruby.
Like my other patch, Emerald Font for FireRed, this does NOT change the font width table, neither does it change the text box graphic, so you can use it safely on in-progress hacks as well as clean roms (provided you did not change the font and/or text box already, of course)
Apply to English Pokemon Emerald BPEE.
I've tested it on clean ROMs and hacks without problems, but make a backup, just in case.
Screenshots and patches are attached.
Credit not required (these were really, really simple to make) but appreciated. :)