Fix Color description in Theme

Fix typo
Allow loading themes from file multiple times (e.g. for party modes)

Output/Themes/Ambient.xml

@@ -11,6 +11,6 @@
     <Skin>Cursor</Skin>
     <W>40,000</W>
     <H>40,000</H>
-    <Color>StaticColor</Color>
+    <Color><Name>StaticColor</Name></Color>
   </Cursor>
 </root>

Vocaluxe/Base/ThemeSystem/CSkin.cs

@@ -108,7 +108,7 @@ namespace Vocaluxe.Base.ThemeSystem
             }
 
             // load colors
-            if (_LoadColors(xmlReader))
+            if (!_LoadColors(xmlReader))
                 return false;
 
             _IsLoaded = true;

Vocaluxe/Base/ThemeSystem/CThemes.cs

@@ -118,8 +118,6 @@ namespace Vocaluxe.Base.ThemeSystem
 
         public static bool ReadThemesFromFolder(string path, int partyModeID)
         {
-            Debug.Assert(_Themes.Count == 0);
-
             List<string> files = CHelper.ListFiles(path, "*.xml", false, true);
 
             List<CTheme> newThemes = new List<CTheme>();
@@ -175,7 +173,8 @@ namespace Vocaluxe.Base.ThemeSystem
 
         public static CTextureRef GetSkinTexture(string textureName, int partyModeID)
         {
-            Debug.Assert(!String.IsNullOrEmpty(textureName));
+            if (String.IsNullOrEmpty(textureName))
+                return null;
             CTheme theme = _GetTheme(partyModeID);
             CTextureRef texture = theme.CurrentSkin.GetTexture(textureName);
             if (texture == null && partyModeID >= 0)