plugin.kts

import com.intellij.application.subscribe
import com.intellij.ide.ui.LafManagerListener
import com.intellij.ui.Gray
import com.intellij.ui.JBColor
import com.intellij.util.ui.JBInsets
import javax.swing.UIManager

fun fixUI() {
	// You can find more constants in:
	//  - https://github.com/JetBrains/intellij-community/blob/master/platform/util/ui/src/com/intellij/util/ui/JBUI.java
	//  - https://github.com/JetBrains/intellij-community/blob/master/platform/platform-resources/src/themes/metadata/IntelliJPlatform.themeMetadata.json
	//  - https://github.com/JetBrains/intellij-community/blob/master/platform/platform-resources/src/themes/metadata/JDK.themeMetadata.json
	UIManager.put("RunWidget.background", Gray._55)
	UIManager.put("RunWidget.runningBackground", Gray._55)
	UIManager.put("ToolWindow.Header.height", 30)
	UIManager.put("ToolWindow.Header.inactiveBackground", JBColor(Gray._244, Gray._55))
	UIManager.put("EditorTabs.tabInsets", JBInsets.create(0, 10))
}

LafManagerListener.TOPIC.subscribe(pluginDisposable, LafManagerListener { fixUI() })

There is no easy answer anymore but finding JComponent and changing its background is always an option 🙈

import com.intellij.openapi.actionSystem.impl.ActionButtonWithText
import com.intellij.openapi.project.Project
import com.intellij.openapi.project.ProjectManagerListener
import com.intellij.openapi.wm.WindowManager
import com.intellij.ui.ComponentUtil
import com.intellij.ui.JBColor
import java.awt.Color
import javax.swing.JComponent

fun fixUI() {
    WindowManager.getInstance()
        .allProjectFrames.map { it.component }
        .flatMap {
            // Lookup via class name because MainToolbar is internal
            // See https://github.com/JetBrains/intellij-community/blob/b1a1086a1c762bd7c0683a87f371c9d57e34588e/platform/platform-impl/src/com/intellij/openapi/wm/impl/headertoolbar/MainToolbar.kt#L79
            val toolbarClass = this.javaClass.classLoader.loadClass("com.intellij.openapi.wm.impl.headertoolbar.MainToolbar")
            ComponentUtil.findComponentsOfType(it, toolbarClass as Class<JComponent>)
        }
        .flatMap {
            ComponentUtil.findComponentsOfType(it, ActionButtonWithText::class.java).filterNotNull()
        }
        .forEach {
            it.background = JBColor(Color(65, 104, 207), Color(65, 104, 207))
        }
}

liveplugin.registerProjectListener(pluginDisposable, object : ProjectManagerListener {
    override fun projectOpened(project: Project) = fixUI()
})