In Code

XAML (preferred)¶

<ContentPagex:Class="...">
<ContentPage.Resources>
<StyleSheetSource="appresources/style.css"/>
</ContentPage.Resources>
</ContentPage>

the Source argument takes an Uri relative to the current xaml control, or relative to the application root if it starts with a /. The style.css has to be an EmbeddedResource.

alternatively, you can inline your style in a CDATA Section

<ContentPagex:Class="...">
<ContentPage.Resources>
<StyleSheet>
<![CDATA[
^contentpage {
 background-color: orange;
 padding: 20;
}
stacklayout > * {
 margin: 3;
}
]]>
</StyleSheet>
</ContentPage.Resources>
</ContentPage>

do not abuse that second syntax.

in C¶

From an embedded resource:

myPage.Resources.Add(StyleSheet.FromAssemblyResource(this.GetType().Assembly,"resource.id.of.the.css"));

or from a TextReader:

using(varreader=newStringReader(my_css_string))
myPage.Resources.Add(StyleSheet.FromReader(reader));

StyleSheet, XamlC and other potential optimizations¶

At this time, CSS StyleSheets are parsed and evaluated at runtime. That aren't compiled. Every time a StyleSheet is used, it's reparsed again. If parsing time is an issue, enabling caching is trivial, but comes at memory cost.