Change the divider height and color to be more consistent.

Signed-off-by: Adam Treat <treat.adam@gmail.com>

gpt4all-chat/qml/Theme.qml

@@ -195,14 +195,7 @@ QtObject {
     }
 
     property color conversationDivider: {
-        switch (MySettings.chatTheme) {
-            case "LegacyDark":
-                return blue900;
-            case "Dark":
-                return darkgray100;
-            default:
-                return gray100;
-        }
+        return dividerColor;
     }
 
     property color settingsDivider: {