Added CSS to better style the electron menubar

src/assets/less/app.less

@@ -0,0 +1,17 @@
+// Theming specific to the electron app
+
+.menubar-menu-container {
+    box-shadow: 0.5px 1px 4px 0px rgba(0,0,0,0.3) !important;
+}
+
+.menubar-menu-container > * {
+    box-shadow: none !important; 
+}
+
+.menubar-menu-container .action-menu-item, .menubar-menu-container .action-label {
+    cursor: default
+}
+
+.menubar-menu-container a:hover {
+    text-decoration: none;
+}

src/assets/less/theme.less

@@ -4,6 +4,9 @@
 // Highlight.js
 @import "./highlight.less";
 
+// Electron app theming
+@import "./app.less";
+
 // Custom OpenFlexure theming
 @global-primary-background: #C32280;