Started adding styles resources.

That could be used by controls. However, we can't apply a StyleResource
to a non-control as we have no reference to our position in the logical
tree. Blocked by OmniXaml issue #80.

src/Perspex.Themes.Default/Accents/BaseLight.paml

@@ -0,0 +1,33 @@
+<Style xmlns="https://github.com/perspex"
+       xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+       xmlns:mut="https://github.com/perspex/mutable">
+  <Style.Resources>
+    <Color x:Key="SystemAltHighColor">#FF000000</Color>
+    <Color x:Key="SystemAltLowColor">#33000000</Color>
+    <Color x:Key="SystemAltMediumColor">#99000000</Color>
+    <Color x:Key="SystemAltMediumHighColor">#CC000000</Color>
+    <Color x:Key="SystemAltMediumLowColor">#66000000</Color>
+    <Color x:Key="SystemBaseHighColor">#FFFFFFFF</Color>
+    <Color x:Key="SystemBaseLowColor">#33FFFFFF</Color>
+    <Color x:Key="SystemBaseMediumColor">#99FFFFFF</Color>
+    <Color x:Key="SystemBaseMediumHighColor">#CCFFFFFF</Color>
+    <Color x:Key="SystemBaseMediumLowColor">#66FFFFFF</Color>
+    <Color x:Key="SystemChromeAltLowColor">#FFF2F2F2</Color>
+    <Color x:Key="SystemChromeBlackHighColor">#FF000000</Color>
+    <Color x:Key="SystemChromeBlackLowColor">#33000000</Color>
+    <Color x:Key="SystemChromeBlackMediumLowColor">#66000000</Color>
+    <Color x:Key="SystemChromeBlackMediumColor">#CC000000</Color>
+    <Color x:Key="SystemChromeDisabledHighColor">#FF333333</Color>
+    <Color x:Key="SystemChromeDisabledLowColor">#FF858585</Color>
+    <Color x:Key="SystemChromeHighColor">#FF767676</Color>
+    <Color x:Key="SystemChromeLowColor">#FF171717</Color>
+    <Color x:Key="SystemChromeMediumColor">#FF1F1F1F</Color>
+    <Color x:Key="SystemChromeMediumLowColor">#FF2B2B2B</Color>
+    <Color x:Key="SystemChromeWhiteColor">#FFFFFFFF</Color>
+    <Color x:Key="SystemListLowColor">#19FFFFFF</Color>
+    <Color x:Key="SystemListMediumColor">#33FFFFFF</Color>
+
+    <mut:SolidColorBrush x:Key="SystemControlBackgroundBaseHighBrush" Color="{StyleResource SystemBaseHighColor}"/>
+    <mut:SolidColorBrush x:Key="SystemControlBackgroundBaseLowBrush" Color="{StyleResource SystemBaseLowColor}"/>
+  </Style.Resources>
+</Style>

src/Perspex.Themes.Default/DefaultTheme.paml

@@ -1,4 +1,6 @@
 <Styles xmlns="https://github.com/perspex">
+  <StyleInclude Source="resm:Perspex.Themes.Default.Accents.BaseLight.paml?assembly=Perspex.Themes.Default"/>
+
   <StyleInclude Source="resm:Perspex.Themes.Default.FocusAdorner.paml?assembly=Perspex.Themes.Default"/>
   <StyleInclude Source="resm:Perspex.Themes.Default.Button.paml?assembly=Perspex.Themes.Default"/>
   <StyleInclude Source="resm:Perspex.Themes.Default.Carousel.paml?assembly=Perspex.Themes.Default"/>

src/Perspex.Themes.Default/Perspex.Themes.Default.csproj

@@ -100,6 +100,9 @@
     </Reference>
   </ItemGroup>
   <ItemGroup>
+    <EmbeddedResource Include="Accents\BaseLight.paml">
+      <SubType>Designer</SubType>
+    </EmbeddedResource>
     <None Include="app.config" />
     <EmbeddedResource Include="Button.paml">
       <SubType>Designer</SubType>