Defaulting the tips theme

[parar020100]

Apparently, using a non-existent theme as an extension default can cause this:

**[TARDIS] lua/tardis/sh_visgui_themes.lua:73: Attempt to access non-existing themedefault
  1. error - [C]:-1
   2. GetGUIThemeElement - lua/tardis/sh_visgui_themes.lua:73
    3. LoadScreenUI - lua/tardis/sh_screen_ui.lua:306
     4. LoadScreen - lua/tardis/sh_screen_ui.lua:767
      5. LoadScreens - lua/entities/gmod_tardis_interior/modules/sh_interior_screens.lua:75
       6. v - lua/entities/gmod_tardis_interior/modules/sh_interior_screens.lua:90
        7. CallHook - lua/entities/gmod_tardis_interior/shared.lua:49
         8. func - lua/entities/gmod_door_exterior/cl_init.lua:38
          9. unknown - lua/includes/extensions/net.lua:38**

Needs checking