oh right, there are some color definitions in the xml files too.
Edit: Here, use the attached gz file and do the following:
Start over with current Melina TWRP initrd untouched
extract it, and change directory to "twres" under the initrd
reverse apply the attached gz patch (either gunzip it or something like zcat ~/melina_theme.diff.gz | patch -R -p1 while in the "twres" dir.
if you did it right (I tested locally), it should patch all the xml and png files successfully (see here)
You now have default TWRP theme (with images and xml) without losing Melina features.
I don't change the theme files often, if at all, so this patch likely works across all older, and even probably some future releases of Melina TWRP, unless a theme change comes from upstream.