Silverlight Dynamic Themes

<ResourceDictionary
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:navigation="clr-namespace:System.Windows.Controls;assembly=System.Windows.Controls.Navigation" 
    xmlns:my="clr-namespace:System;assembly=mscorlib" >

    <ResourceDictionary.MergedDictionaries>
        <ResourceDictionary Source="Brushes.xaml"/>
    </ResourceDictionary.MergedDictionaries>
    
    <FontFamily x:Key="HeaderFontFamily">Segoe UI Light, Lucida Sans Unicode, Verdana</FontFamily>
    <FontFamily x:Key="ContentFontFamily">Segoe UI, Lucida Sans Unicode, Verdana</FontFamily>

    <!-- Font Sizes. -->
    <my:Double x:Key="DefaultFontSize">11</my:Double>
    <my:Double x:Key="InputFontSize">10</my:Double>
    <my:Double x:Key="NormalFontSize">10</my:Double>
    <my:Double x:Key="Heading1FontSize">34</my:Double>
    <my:Double x:Key="Heading2FontSize">22</my:Double>
    <my:Double x:Key="Heading3FontSize">19</my:Double>
    <my:Double x:Key="Heading4FontSize">16</my:Double>
    <my:Double x:Key="Heading5FontSize">13</my:Double>
    <my:Double x:Key="Heading6FontSize">9</my:Double>

    <my:Double x:Key="ApplicationNameFontSize">48</my:Double>
    <my:Double x:Key="HeaderFontSize">18.333</my:Double>
    <my:Double x:Key="NavigationFontSize">26</my:Double>

    <my:Double x:Key="ContentFontSize">12</my:Double>
    <my:Double x:Key="HyperlinkFontSize">12</my:Double>
    <my:Double x:Key="LabelFontSize">10</my:Double>
    <my:Double x:Key="ControlTitleFontSize">13</my:Double>
    <my:Double x:Key="ControlTitleBigFontSize">13</my:Double>
    <my:Double x:Key="ControlContentFontSize">9</my:Double>


</ResourceDictionary>