improve styling of preferences side menu (#4556)

Author: matthiasgeiger

src/main/java/org/jabref/gui/preferences/PreferencesDialog.css

@@ -1,5 +1,7 @@
 #sideMenu {
     -fx-background-color: -jr-white;
+    -fx-border-color: -fx-outer-border;
+    -fx-border-width: 1;
 }
 
 #sideMenu > .virtual-flow > .clipped-container > .sheet > .list-cell {

src/main/java/org/jabref/gui/preferences/PreferencesDialog.java

@@ -116,20 +116,21 @@ private void construct() {
 
         VBox buttonContainer = new VBox();
         buttonContainer.setAlignment(Pos.BOTTOM_LEFT);
+        buttonContainer.setSpacing(3.0);
         Button importPreferences = new Button(Localization.lang("Import preferences"));
         importPreferences.setTooltip(new Tooltip(Localization.lang("Import preferences from file")));
         importPreferences.setOnAction(e -> importPreferences());
-        importPreferences.getStyleClass().add("text-button");
+        importPreferences.setMaxWidth(Double.MAX_VALUE);
         Button exportPreferences = new Button(Localization.lang("Export preferences"));
         exportPreferences.setTooltip(new Tooltip(Localization.lang("Export preferences to file")));
         exportPreferences.setOnAction(e -> exportPreferences());
-        exportPreferences.getStyleClass().add("text-button");
+        exportPreferences.setMaxWidth(Double.MAX_VALUE);
         Button showPreferences = new Button(Localization.lang("Show preferences"));
         showPreferences.setOnAction(e -> new PreferencesFilterDialog(new JabRefPreferencesFilter(prefs)).setVisible(true));
-        showPreferences.getStyleClass().add("text-button");
+        showPreferences.setMaxWidth(Double.MAX_VALUE);
         Button resetPreferences = new Button(Localization.lang("Reset preferences"));
         resetPreferences.setOnAction(e -> resetPreferences());
-        resetPreferences.getStyleClass().add("text-button");
+        resetPreferences.setMaxWidth(Double.MAX_VALUE);
         buttonContainer.getChildren().addAll(
                 importPreferences,
                 exportPreferences,
@@ -138,6 +139,7 @@ private void construct() {
         );
 
         VBox spacer = new VBox();
+        spacer.setPrefHeight(10.0);
         VBox.setVgrow(tabsList, Priority.ALWAYS);
         VBox.setVgrow(spacer, Priority.SOMETIMES);
         vBox.getChildren().addAll(