If this works then I'm all good with it btw, I only gave the other suggestion in case that wouldn't work.
Just remember that this (in \Assets\XML\Art\CIV4ArtDefines_Misc.xml):
<MiscArtInfo>
<Type>DEFAULT_THEME_NAME</Type>
<Path>Mods/Caveman2Cosmos/Resource/C2C.thm</Path>
<fScale>0</fScale>
<NIF/>
<KFM/>
</MiscArtInfo>
needs to point to our /Resource folder, so we will have to add a "Mod" in its path, or move the "Resource" folder out to the same folder level as the "Source" folder.
I guess it belongs in the "Mod" folder so "Mods/Caveman2Cosmos/Mod/Resource/C2C.thm" is the method I prefer.