Editor theme changes on every load

Please share screenshots of Settings | Editor | Color Scheme page before and after restart, screenshot of the message plus your idea.log (Help > Show Log in ...)

The before screen shoot


This is after reload


This is the errors I get



This is the stack trace of the IDE error

com.intellij.diagnostic.PluginException: ID "MTDarkerTheme" is already taken by action "Darker Theme (Material darker)" (Plugin: Material Theme UI Lite). Action "Darker Theme (Material darker)" (Plugin: Material Theme UI) cannot use the same ID [Plugin: com.chrisrm.idea.MaterialThemeUI]
at com.intellij.openapi.actionSystem.impl.ActionManagerImpl.reportActionError(ActionManagerImpl.java:389)
at com.intellij.openapi.actionSystem.impl.ActionManagerImpl.reportActionError(ActionManagerImpl.java:384)
at com.intellij.openapi.actionSystem.impl.ActionManagerImpl.registerChameleon(ActionManagerImpl.java:1308)
at com.intellij.openapi.actionSystem.impl.ActionManagerImpl.addToMap(ActionManagerImpl.java:1278)
at com.intellij.openapi.actionSystem.impl.ActionManagerImpl.registerAction(ActionManagerImpl.java:1247)
at com.intellij.openapi.actionSystem.impl.ActionManagerImpl.registerOrReplaceActionInner(ActionManagerImpl.java:732)
at com.intellij.openapi.actionSystem.impl.ActionManagerImpl.processActionElement(ActionManagerImpl.java:705)
at com.intellij.openapi.actionSystem.impl.ActionManagerImpl.processGroupElement(ActionManagerImpl.java:859)
at com.intellij.openapi.actionSystem.impl.ActionManagerImpl.registerPluginActions(ActionManagerImpl.java:517)
at com.intellij.openapi.actionSystem.impl.ActionManagerImpl.registerActions(ActionManagerImpl.java:197)
at com.intellij.openapi.actionSystem.impl.ActionManagerImpl.<init>(ActionManagerImpl.java:174)
at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:490)
at com.intellij.serviceContainer.ConstructorInjectionKt.instantiateUsingPicoContainer(constructorInjection.kt:47)
at com.intellij.serviceContainer.ComponentManagerImpl.instantiateClassWithConstructorInjection(ComponentManagerImpl.kt:771)
at com.intellij.serviceContainer.ServiceComponentAdapter.createAndInitialize(ServiceComponentAdapter.kt:49)
at com.intellij.serviceContainer.ServiceComponentAdapter.access$createAndInitialize(ServiceComponentAdapter.kt:13)
at com.intellij.serviceContainer.ServiceComponentAdapter$doCreateInstance$1.run(ServiceComponentAdapter.kt:43)
at com.intellij.openapi.progress.impl.CoreProgressManager.registerIndicatorAndRun(CoreProgressManager.java:688)
at com.intellij.openapi.progress.impl.CoreProgressManager.executeProcessUnderProgress(CoreProgressManager.java:634)
at com.intellij.openapi.progress.impl.ProgressManagerImpl.executeProcessUnderProgress(ProgressManagerImpl.java:64)
at com.intellij.openapi.progress.impl.CoreProgressManager.executeNonCancelableSection(CoreProgressManager.java:218)
at com.intellij.serviceContainer.ServiceComponentAdapter.doCreateInstance(ServiceComponentAdapter.kt:42)
at com.intellij.serviceContainer.BaseComponentAdapter.getInstanceUncached(BaseComponentAdapter.kt:110)
at com.intellij.serviceContainer.BaseComponentAdapter.getInstance(BaseComponentAdapter.kt:64)
at com.intellij.serviceContainer.ComponentManagerImpl.doGetService(ComponentManagerImpl.kt:488)
at com.intellij.serviceContainer.ComponentManagerImpl.getService(ComponentManagerImpl.kt:471)
at com.intellij.openapi.actionSystem.ActionManager.getInstance(ActionManager.java:30)
at com.intellij.openapi.actionSystem.impl.ActionPreloader.preload(ActionPreloader.java:14)
at com.intellij.openapi.application.Preloader.lambda$preload$0(Preloader.java:84)
at com.intellij.openapi.progress.impl.CoreProgressManager.lambda$runProcess$2(CoreProgressManager.java:178)
at com.intellij.openapi.progress.impl.CoreProgressManager.registerIndicatorAndRun(CoreProgressManager.java:688)
at com.intellij.openapi.progress.impl.CoreProgressManager.executeProcessUnderProgress(CoreProgressManager.java:634)
at com.intellij.openapi.progress.impl.ProgressManagerImpl.executeProcessUnderProgress(ProgressManagerImpl.java:64)
at com.intellij.openapi.progress.impl.CoreProgressManager.runProcess(CoreProgressManager.java:165)
at com.intellij.openapi.application.Preloader.lambda$preload$1(Preloader.java:74)
at com.intellij.util.concurrency.BoundedTaskExecutor.doRun(BoundedTaskExecutor.java:216)
at com.intellij.util.concurrency.BoundedTaskExecutor.access$200(BoundedTaskExecutor.java:27)
at com.intellij.util.concurrency.BoundedTaskExecutor$1.execute(BoundedTaskExecutor.java:195)
at com.intellij.util.ConcurrencyUtil.runUnderThreadName(ConcurrencyUtil.java:213)
at com.intellij.util.concurrency.BoundedTaskExecutor$1.run(BoundedTaskExecutor.java:184)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
at java.base/java.util.concurrent.Executors$PrivilegedThreadFactory$1$1.run(Executors.java:668)
at java.base/java.util.concurrent.Executors$PrivilegedThreadFactory$1$1.run(Executors.java:665)
at java.base/java.security.AccessController.doPrivileged(Native Method)
at java.base/java.util.concurrent.Executors$PrivilegedThreadFactory$1.run(Executors.java:665)
at java.base/java.lang.Thread.run(Thread.java:834)

Here is the log https://drive.google.com/file/d/1QRPmQXXh-6EHPftu_06J4PlrfukHXkI1/view?usp=sharing

Looks like themes conflict. You have a lot of custom themes installed -

Solarized Themes (2.3.0), Halcyon Theme (2.0.0), Vuesion Theme (1.9.2), 
Hiberbee Theme (20210127181309), Cyan Light Theme (1.2), 
Dark Purple Theme (1.2), Atom Material Icons (36.0), 
Material Theme UI Lite (13.1), One Dark theme (5.1.5), 
Visual Studio Code Dark Plus Theme (2.3), 
Solarized Theme (3.0.0), Dracula Theme (1.11.3), Gruvbox Theme (0.4.4),  
Material Theme UI (6.2.3), Spacegray Theme (1.4.1) 

and looks as if at least some of them (Material Theme UI Lite and Material Theme UI in particular) share same IDs and thus cause conflicts. I'd suggest reporting the issue to the plugin vendor

Meanwhile try disabling/uninstalling those themes that aren't used and see if the issue persists

So out of two material themes, Material Theme UI Light and Material Theme UI, I removed the light one and it fixed the IDE error. But the editor's theme still changed after reload.

Another issue that I have is, prettier stops working from time to time. Also I am keep getting on some constants "Unused constant MyConst", even though they are clearly used. 

False 'unused' violations can be caused by broken indexes. Please try invalidating caches (see https://youtrack.jetbrains.com/issue/IDEA-266391) - does the issue persist?

Clearing the cache seems to fix the false violation issue.