client/state/themes/actions.js
function suffixThemeIdForInstall( state, siteId, themeId ) {
	// AT sites do not use the -wpcom suffix
	if ( isSiteAutomatedTransfer( state, siteId ) ) {
		return themeId;
	}
	if ( ! isDownloadableFromWpcom( state, themeId ) ) {
		return themeId;
	}
	return themeId + '-wpcom';
}
function suffixThemeIdForInstall( state, siteId, themeId ) {
	// AT sites do not use the -wpcom suffix
	if ( isSiteAutomatedTransfer( state, siteId ) ) {
		return themeId;
	}
	if ( ! isDownloadableFromWpcom( state, themeId ) ) {
		return themeId;
	}
	return themeId + '-wpcom';
}
Install of any theme hosted as a zip on wpcom must
be suffixed with -wpcom. Themes on AT sites are not
installed via downloaded zip.

@param {Object} state Global state tree
@param {number} siteId Site ID
@param {string} themeId Theme ID
@return {string} the theme id to use when installing the theme