Reset usertheme to default if deleted #868

Job	Run time
build	3m 10s
Julia 1.6 - ubuntu-latest - x64	3m 8s
Julia 1.6 - macOS-latest - x64	4m 40s
Julia 1.6 - windows-latest - x64	15m 21s
Julia 1 - ubuntu-latest - x64	5m 2s
Julia 1 - macOS-latest - x64	7m 57s
Julia 1 - windows-latest - x64	5m 38s
Julia nightly - ubuntu-latest - x64	5m 16s
Julia nightly - macOS-latest - x64	5m 31s
Julia nightly - windows-latest - x64	5m 53s
	1h 1m 36s

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