docs: update theming config example #572

Job	Run time
run_ci (ubuntu-20.04, 3.11, discovery+lms+forum)	22m 17s
run_ci (ubuntu-20.04, 3.11, registrar+lms)	16m 49s
run_ci (ubuntu-20.04, 3.11, ecommerce+lms)	21m 19s
run_ci (ubuntu-20.04, 3.11, edx_notes_api+lms)	17m 13s
run_ci (ubuntu-20.04, 3.11, credentials+lms)	18m 11s
run_ci (ubuntu-20.04, 3.11, xqueue)	2m 51s
run_ci (ubuntu-20.04, 3.11, analyticsapi+insights+lms)	23m 15s
run_ci (ubuntu-20.04, 3.11, enterprise-catalog+lms)	23m 22s
run_ci (ubuntu-20.04, 3.11, license-manager+lms)	17m 47s
	2h 43m 4s

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