Add syntax highlighting for .css files.

src/Libraries/AvalonEdit/ICSharpCode.AvalonEdit/Highlighting/Resources/CSS-Mode.xshd

@@ -0,0 +1,57 @@
+<SyntaxDefinition name="CSS" extensions=".css" xmlns="http://icsharpcode.net/sharpdevelop/syntaxdefinition/2008">
+	<Color name="Comment" foreground="Gray" />
+	<Color name="String" foreground="Green" />
+	<Color name="Selector" foreground="DarkBlue" fontWeight="bold" />
+	<Color name="Class" foreground="DarkMagenta" />
+	<Color name="Property" foreground="Red"/>
+	<Color name="Value" foreground="Blue" />
+	<Color name="Default" foreground="Pink" />
+	<Color name="CurlyBraces" foreground="Black" />
+	<Color name="Colon" foreground="Black" />
+	<RuleSet ignoreCase="true">
+		<Span color="Comment" multiline="true">
+			<Begin>/\*</Begin>
+			<End>\*/</End>
+		</Span>
+		<Span
+			color="Property"
+			multiline="true"
+			ruleSet="CSSBlock">
+			<Begin color="CurlyBraces">\{</Begin>
+			<End color="CurlyBraces">\}</End>
+		</Span>
+		<Span color="Class">
+			<Begin>\#</Begin>
+			<End>\s</End>
+		</Span>
+		<Rule color="Selector">[\d\w]</Rule>
+	</RuleSet>
+
+	<RuleSet name="CSSBlock">
+		<Span color="Comment" multiline="true">
+			<Begin>/\*</Begin>
+			<End>\*/</End>
+		</Span>
+		<Span color="Value" multiline="true">
+			<Begin color="Colon">\:</Begin>
+			<End color="CurlyBraces">\;|(?=\})</End>
+			<RuleSet>
+				<Span color="String" multiline="true">
+					<Begin>"</Begin>
+					<End>"</End>
+					<RuleSet>
+						<Span begin="\\" end="." />
+					</RuleSet>
+				</Span>
+				<Span color="String" multiline="true">
+					<Begin>'</Begin>
+					<End>'</End>
+					<RuleSet>
+						<Span begin="\\" end="." />
+					</RuleSet>
+				</Span>
+			</RuleSet>
+		</Span>
+	</RuleSet>
+</SyntaxDefinition>
+

src/Libraries/AvalonEdit/ICSharpCode.AvalonEdit/Highlighting/Resources/Resources.cs

@@ -29,6 +29,7 @@ namespace ICSharpCode.AvalonEdit.Highlighting
 			
 			hlm.RegisterHighlighting("Boo", new[] { ".boo" }, "Boo.xshd");
 			hlm.RegisterHighlighting("Coco", new[] { ".atg" }, "Coco-Mode.xshd");
+			hlm.RegisterHighlighting("CSS", new[] { ".css" }, "CSS-Mode.xshd");
 			hlm.RegisterHighlighting("C++", new[] { ".c", ".h", ".cc", ".cpp" , ".hpp" }, "CPP-Mode.xshd");
 			hlm.RegisterHighlighting("Java", new[] { ".java" }, "Java-Mode.xshd");
 			hlm.RegisterHighlighting("Patch", new[] { ".patch", ".diff" }, "Patch-Mode.xshd");

src/Libraries/AvalonEdit/ICSharpCode.AvalonEdit/ICSharpCode.AvalonEdit.csproj

@@ -409,4 +409,7 @@
     <Page Include="TextEditor.xaml" />
     <Page Include="themes\generic.xaml" />
   </ItemGroup>
+  <ItemGroup>
+    <EmbeddedResource Include="Highlighting\Resources\CSS-Mode.xshd" />
+  </ItemGroup>
 </Project>