docs: update css variables info #4494

Job	Run time
build	4m 59s
setup-matrix	5s
lint	3m 33s
test	1m 53s
e2e (1, ../ui/collections/forms/src/CheckboxFieldFF/features/can_toggle_a_boolean.feature, ../ui/...	5m 48s
e2e (2, ../ui/collections/forms/src/CheckboxFieldFF/features/can_toggle_a_value.feature, ../ui/co...	5m 28s
e2e (3, ../ui/collections/forms/src/CheckboxFieldFF/features/displays_error.feature, ../ui/collec...	5m 38s
e2e (4, ../ui/collections/forms/src/FileInputFieldFF/features/accepts_file.feature, ../ui/collect...	5m 28s
e2e (5, ../ui/collections/forms/src/FileInputFieldFF/features/displays_error.feature, ../ui/colle...	5m 43s
e2e (6, ../ui/collections/forms/src/InputFieldFF/features/can_set_a_value.feature, ../ui/collecti...	5m 54s
e2e (7, ../ui/collections/forms/src/InputFieldFF/features/displays_error.feature, ../ui/collectio...	5m 27s
publish	1m 20s
	51m 16s

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