GUI elements not showing in Android and Web player

Hello everyone! The problem I have is that no GUI element shows when I test my app in Web player and on an Android device. It all works fine in Unity itself. When I select Windows platform and press Build and Run it all works great, but when I only build it and after that start the exe file all GUI elements dont't show again. To be precise, I have a script in which some GUILayout elements are defined. I also tried with GUI.Label and it was the same situation. I have defined 2 GUIStyle elements which I use for buttons styles. They had some styling like font color, texture and size. At this moment they are all black, without textures and borders, the font is with normal style and using default size. It still doesn't work.

Edit: I tried adding GUI text element from GameObject -> Create other -> GUI text and is shows in all players. So I still think the problem is with the GUIStyle, but I don't know exactly what is wrong.

This is my code:

 public class MainMenu : MonoBehaviour {
     
     float standartButtonHeight = Screen.height/10;
     float minButtonHeight = 10;
     float itemButtonHeight;
     float itemButtonWidth = Screen.width;
     bool showMenuItems = false;
     bool showMenuButton = true;
     bool showMuseumsMenu = false;
     bool showChurchesMenu = false;
     bool showMemorialsMenu = false;
     bool showStreetsMenu = false;
     bool showOtherMenu = false;
 
     public GUIStyle mainButtonStyle;
     public GUIStyle buttonStyle_2;
 
     void OnGUI()
     {    
         
         if (standartButtonHeight < minButtonHeight)
         {
             itemButtonHeight = minButtonHeight;
         } else 
             {
             itemButtonHeight = standartButtonHeight;
             }
 
         if(showMenuButton)
         {
             if (GUI.Button (new Rect (0, 0, itemButtonWidth/3, itemButtonHeight), "Menu", buttonStyle_2)) 
             {
                 showMenuButton = false;
                 showMenuItems = true;
             }
         }
         ShowMainMenu ();    
         MuseumsMenu ();
         ChurchesMenu();
 
     }
 
     void ShowMainMenu()
     {
         if(showMenuItems)
         {            
             GUILayout.BeginVertical();
             
             GUILayout.Button ("About Sofia", buttonStyle_2, GUILayout.Width(itemButtonWidth), GUILayout.Height(itemButtonHeight));
             if (GUILayout.Button ("Museums", buttonStyle_2, GUILayout.Width(itemButtonWidth), GUILayout.Height(itemButtonHeight)))
             {
                 showMuseumsMenu = true;
                 showMenuItems = false;
                 MuseumsMenu();
 
             }
             
             //few more buttons defined in the same way
             
 if (GUILayout.Button ("Exit", buttonStyle_2))
             {
                 Application.Quit();
             }
             GUILayout.EndVertical();
             
         }
 
 
 
     }