Add /theme:toggle command #5692

Job	Run time
build-and-check	2m 47s
test-integration-packages/billing	1m 0s
test-integration-packages/internal	59s
test-integration-sdk	40s
test-integration-packages/agent-runtime	49s
test-integration-web	43s
test-integration-cli	42s
test-integration-common	46s
test-web	44s
test-.agents	49s
test-cli	50s
test-packages/billing	46s
test-common	38s
test-packages/agent-runtime	44s
test-packages/internal	1m 2s
test-sdk	42s
	14m 41s

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