UI: Fix qpushbutton menu icon

Closes jp9000/obs-studio#1141

UI/data/themes/Dark.qss

@@ -403,6 +403,12 @@ QPushButton:checked {
     background-color: rgb(122,121,122); /* light */
 }
 
+QPushButton::menu-indicator {
+    image: url(./Dark/down_arrow.png);
+    subcontrol-position: right;
+    subcontrol-origin: padding;
+    width: 25px;
+}
 
 /* Sliders */
 

UI/data/themes/Rachni.qss

@@ -775,6 +775,13 @@ QPushButton:disabled {
 	color: rgb(162, 161, 162); /* Lighter Gray */
 }
 
+QPushButton::menu-indicator {
+    image: url(./Rachni/down_arrow.png);
+    subcontrol-position: right;
+    subcontrol-origin: padding;
+    width: 25px;
+}
+
 /******************************/
 /* --- Dialog Box Buttons --- */
 /******************************/