diff --git a/CHANGELOG.md b/CHANGELOG.md
index be389cfec4d..9924f0ca9b3 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -145,6 +145,8 @@ Note that this project **does not** adhere to [Semantic Versioning](https://semv
 - We fixed an issue where hierarchical keywords would only show the parent keyword in the entry editor. [#11390](https://github.com/JabRef/jabref/issues/11390)
 - We fixed an issue where some file choosers regarding LaTeX-aux files did not open in the directory of the last selected file. [#13861](https://github.com/JabRef/jabref/pull/13861)
 - We fixed an issue where the LaTeX file directory was not stored correctly in combination with the usage of groups from aux files. [#8344](https://github.com/JabRef/jabref/issues/8344)
+- We prevented a brief flash of the default JavaFX (Modena) theme on startup. [#13877](https://github.com/JabRef/jabref/pull/13877)
+- We fixed an issue where button-bar buttons truncated long text with ellipsis. [#13877](https://github.com/JabRef/jabref/pull/13877)
 - We fixed an issue where ignoring of subdirectories via `.gitingore` patterns did not work in the "Find unlinked files dialog". [forum#5425](https://discourse.jabref.org/t/set-list-of-ignored-folders-paths/5425/6)
 
 ### Removed
diff --git a/jabgui/src/main/java/org/jabref/gui/JabRefGUI.java b/jabgui/src/main/java/org/jabref/gui/JabRefGUI.java
index dd5f3f4be1b..b96f08b6f30 100644
--- a/jabgui/src/main/java/org/jabref/gui/JabRefGUI.java
+++ b/jabgui/src/main/java/org/jabref/gui/JabRefGUI.java
@@ -292,7 +292,7 @@ private void openWindow() {
         Scene scene = new Scene(JabRefGUI.mainFrame);
 
         LOGGER.debug("installing CSS");
-        themeManager.installCss(scene);
+        themeManager.installCssImmediately(scene);
 
         LOGGER.debug("Handle TextEditor key bindings");
         scene.addEventFilter(KeyEvent.KEY_PRESSED, event -> {
diff --git a/jabgui/src/main/java/org/jabref/gui/theme/ThemeManager.java b/jabgui/src/main/java/org/jabref/gui/theme/ThemeManager.java
index 37c14da1b13..cd8cde0aefb 100644
--- a/jabgui/src/main/java/org/jabref/gui/theme/ThemeManager.java
+++ b/jabgui/src/main/java/org/jabref/gui/theme/ThemeManager.java
@@ -105,19 +105,29 @@ public ThemeManager(@NonNull WorkspacePreferences workspacePreferences,
         }
     }
 
-    /// Installs the base and additional css files as stylesheets in the given scene.
+    /// Installs the base and additional CSS files as stylesheets in the given scene.
+    ///
+    /// This method is primarily intended to be called by `JabRefGUI` during startup.
+    /// Using `installCss` directly would cause a delay in theme application, resulting
+    /// in a brief flash of the default JavaFX theme (Modena CSS) before the intended theme appears.
+    public void installCssImmediately(Scene scene) {
+        List<String> stylesheets = scene.getStylesheets();
+        scene.getStylesheets().clear();
+        List<String> baseOrThemeStylesheet = Stream
+                .of(baseStyleSheet.getSceneStylesheet(),
+                        theme.getAdditionalStylesheet().map(StyleSheet::getSceneStylesheet).orElse(null)
+                ).filter(Objects::nonNull)
+                .map(URL::toExternalForm)
+                .toList();
+
+        stylesheets.addAll(baseOrThemeStylesheet);
+    }
+
+    /// Registers a runnable on JavaFX thread to install the base and additional css files as stylesheets in the given scene.
     public void installCss(@NonNull Scene scene) {
         // Because of race condition in JavaFX, IndexOutOfBounds will be thrown, despite
         // all the invocation to this method come directly from the UI thread
-        UiTaskExecutor.runInJavaFXThread(() -> {
-            List<String> stylesheets = Stream
-                    .of(baseStyleSheet.getSceneStylesheet(),
-                            theme.getAdditionalStylesheet().map(StyleSheet::getSceneStylesheet).orElse(null)
-                    ).filter(Objects::nonNull)
-                    .map(URL::toExternalForm)
-                    .toList();
-            scene.getStylesheets().setAll(stylesheets);
-        });
+        UiTaskExecutor.runInJavaFXThread(() -> installCssImmediately(scene));
     }
 
     /// Installs the css file as a stylesheet in the given web engine. Changes in the
diff --git a/jabgui/src/main/resources/org/jabref/gui/Base.css b/jabgui/src/main/resources/org/jabref/gui/Base.css
index cdb132504ad..3af664e07bc 100644
--- a/jabgui/src/main/resources/org/jabref/gui/Base.css
+++ b/jabgui/src/main/resources/org/jabref/gui/Base.css
@@ -414,12 +414,6 @@ TextFlow > .tooltip-text-monospaced {
     -fx-padding: 0.5em 1em 0.5em 1em;
 }
 
-.dialog-pane .button-bar .button {
-    -fx-min-width: 6em;
-    -fx-pref-width: 6em;
-    -fx-max-width: 6em;
-}
-
 .menu-button > .label {
     -fx-padding: 0 8 0 8;
 }