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app/gui/src/main/resources/themes/dark-theme.css

@@ -1,3 +1,4 @@
+/* Taken from https://stackoverflow.com/a/58441758/6947285 */
 .root {
     -fx-accent: #1e74c6;
     -fx-focus-color: -fx-accent;

app/gui/src/main/scala/org/bitcoins/gui/WalletGUI.scala

@@ -1,7 +1,7 @@
 package org.bitcoins.gui
 
 import javafx.event.{ActionEvent, EventHandler}
-import org.bitcoins.gui.settings.SettingsPane
+import org.bitcoins.gui.settings.{SettingsPane, Themes}
 import scalafx.application.{JFXApp, Platform}
 import scalafx.beans.property.StringProperty
 import scalafx.geometry.{Insets, Pos}
@@ -104,13 +104,20 @@ object WalletGUI extends JFXApp {
     )
   }
 
-  stage = new JFXApp.PrimaryStage {
-    title = "Bitcoin-S Wallet"
-    scene = new Scene(rootView)
+  private val styleSheets = if (GlobalData.defaultDarkTheme) {
+    Seq(Themes.DarkTheme.fileLocation)
+  } else {
+    Seq.empty
   }
 
-  if (GlobalData.defaultDarkTheme) {
-    stage.scene.value.getStylesheets.add("/themes/dark-theme.css")
+  private val walletScene = new Scene {
+    root = rootView
+    stylesheets = styleSheets
+  }
+
+  stage = new JFXApp.PrimaryStage {
+    title = "Bitcoin-S Wallet"
+    scene = walletScene
   }
 
   private val taskRunner = new TaskRunner(resultArea, glassPane)

app/gui/src/main/scala/org/bitcoins/gui/settings/SettingsPane.scala

@@ -2,7 +2,6 @@ package org.bitcoins.gui.settings
 
 import javafx.event.{ActionEvent, EventHandler}
 import org.bitcoins.gui.GlobalData
-import org.bitcoins.gui.WalletGUI.stage
 import scalafx.scene.control.CheckBox
 import scalafx.scene.layout.StackPane
 
@@ -14,9 +13,9 @@ class SettingsPane {
     onAction = new EventHandler[ActionEvent] {
       override def handle(event: ActionEvent): Unit = {
         if (!selected.value) {
-          stage.scene.value.getStylesheets.removeAll("/themes/dark-theme.css")
+          Themes.DarkTheme.undoTheme
         } else {
-          stage.scene.value.getStylesheets.add("/themes/dark-theme.css")
+          Themes.DarkTheme.applyTheme
         }
         ()
       }

app/gui/src/main/scala/org/bitcoins/gui/settings/Themes.scala

@@ -0,0 +1,26 @@
+package org.bitcoins.gui.settings
+
+import org.bitcoins.gui.WalletGUI.stage
+
+sealed abstract class Theme {
+  def fileLocation: String
+
+  def applyTheme: Boolean =
+    stage.scene.value.getStylesheets.add(fileLocation)
+
+  def undoTheme: Boolean =
+    stage.scene.value.getStylesheets.removeAll(fileLocation)
+}
+
+object Themes {
+
+  final case object DarkTheme extends Theme {
+    override def fileLocation: String = "/themes/dark-theme.css"
+  }
+
+  val all: Vector[Theme] = Vector(DarkTheme)
+
+  def fromString(str: String): Option[Theme] = {
+    all.find(theme => str.toLowerCase() == theme.toString.toLowerCase)
+  }
+}