Move configuration to appearance page

[timja]

Requires:
jenkinsci/jenkins#8403

Screenshots

Testing done

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6. Ensure you have provided tests - that demonstrates feature works or fixes the issue

[basil]

[ERROR] io.jenkins.plugins.darktheme.jcasc.darksystem.DarkSystemThemeJcascTest -- Time elapsed: 23.65 s <<< FAILURE!
java.lang.AssertionError: Unexpected exception 
        at io.jenkins.plugins.casc.misc.JenkinsConfiguredWithCodeRule.before(JenkinsConfiguredWithCodeRule.java:50)
        at org.jvnet.hudson.test.JenkinsRule$1.evaluate(JenkinsRule.java:601)
        at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
        at java.base/java.lang.Thread.run(Thread.java:833)
Caused by: io.jenkins.plugins.casc.ConfiguratorException: unclassified: error configuring 'unclassified' with class io.jenkins.plugins.casc.impl.configurators.GlobalConfigurationCategoryConfigurator configurator
        at io.jenkins.plugins.casc.ConfigurationAsCode.invokeWith(ConfigurationAsCode.java:737)
        at io.jenkins.plugins.casc.ConfigurationAsCode.checkWith(ConfigurationAsCode.java:797)
        at io.jenkins.plugins.casc.ConfigurationAsCode.configureWith(ConfigurationAsCode.java:783)
        at io.jenkins.plugins.casc.ConfigurationAsCode.configureWith(ConfigurationAsCode.java:652)
        at io.jenkins.plugins.casc.ConfigurationAsCode.configure(ConfigurationAsCode.java:621)
        at io.jenkins.plugins.casc.misc.JenkinsConfiguredWithCodeRule.before(JenkinsConfiguredWithCodeRule.java:47)
        ... 3 more
Caused by: io.jenkins.plugins.casc.ConfiguratorException: Invalid configuration elements for type class jenkins.model.GlobalConfigurationCategory$Unclassified : themeManager.
Available attributes : administrativeMonitorsConfiguration, ansiColorBuildWrapper, artifactManager, bitbucketEndpointConfiguration, buildDiscarders, builtInNode, casCGlobalConfig, customFolderIcon, defaultDisplayUrlProvider, defaultFolderConfiguration, defaultView, email-ext, envVarsFilter, fingerprints, gitHubConfiguration, gitHubPluginConfig, gitLabConnectionConfig, gitLabServers, globalConfigFiles, globalConfigurationImpl, globalDefaultFlowDurabilityLevel, globalLibraries, httpRequestGlobalConfig, jacoco, junitTestResultStorage, location, lockableResourcesManager, mailer, mavenModuleSet, metricsAccessKey, myView, nodeProperties, pipeline-model-docker, plugin, pollSCM, projectNamingStrategy, quietPeriod, rebuildDescriptor, resourceRoot, scmGit, scmRetryCount, shell, singleConditionalBuilder, slackNotifier, subversionSCM, throttleJobProperty, timestamper, upstream, usageStatistics, viewsTabBar
        at io.jenkins.plugins.casc.BaseConfigurator.handleUnknown(BaseConfigurator.java:387)
        at io.jenkins.plugins.casc.BaseConfigurator.configure(BaseConfigurator.java:374)
        at io.jenkins.plugins.casc.BaseConfigurator.check(BaseConfigurator.java:293)
        at io.jenkins.plugins.casc.ConfigurationAsCode.lambda$checkWith$9(ConfigurationAsCode.java:797)
        at io.jenkins.plugins.casc.ConfigurationAsCode.invokeWith(ConfigurationAsCode.java:731)
        ... 8 more