Why do you want to change them in the first place? If you can read XML, it shouldn't be too hard to spot them, as they're only a couple of lines down.
Here's where (I'm using America as an example):
<CivilizationArtInfo>
<Type>ART_DEF_CIVILIZATION_AMERICA</Type>
<Button>,Art/Interface/Buttons/Civilizations/America.dds,Art/Interface/Buttons/Civics_Civilizations_Religions_Atlas.dds,1,5</Button>
<Path>Art/Interface/TeamColor/FlagDECAL_Star.dds</Path>
<bWhiteFlag>0</bWhiteFlag>
</CivilizationArtInfo>
Unless you're modifying a python file (I'd have to check which one. It might be hard coded), you're going to have to replace a civilization. I don't see what was so hard about finding that.