Added some documentation for styles.

docs/styles.md

@@ -0,0 +1,93 @@
+# Styling in Perspex
+
+The main difference between Perspex and existing XAML toolkits such as WPF and
+UWP is in styling. Styling in Perspex uses a CSS-like system that aims to be
+more powerful and flexible than existing XAML styling systems. For convenience
+for the rest of this document we'll refer to existing XAML toolkit's styling as
+"WPF styling" as that's where it originated.
+
+## Basics
+
+- Styles are defined on the `Control.Styles` collection (as opposed to in
+`ResourceDictionaries` in WPF).
+- Styles have a `Selector` and a collection of `Setter`s
+- Selector works like a CSS selector.
+- Setters function like WPF's setters.
+- Styles are applied to a control and all its descendants, depending on whether
+  the selector matches.
+
+## Simple example
+
+Make all `Button`s in a `StackPanel` have a blue `Background`:
+
+    <StackPanel>
+      <StackPanel.Styles>
+        <Style Selector="Button">
+          <Setter Property="Button.Background" Value="Blue"/>
+        </Style>
+      </StackPanel.Styles>
+      <Button>I will have a blue background.</Button>
+    </StackPanel>
+
+This is very similar to WPF, except `TargetType` is replaced by `Selector`.
+
+*Note that currently (as of Alpha 2) you **always** need to specify the fully
+qualified property name (i.e. `Button.Background` instead of simply
+`Background`). This restriction will be lifted in future.*
+
+## Style Classes
+
+As in CSS, controls can be given *style classes* which can be used in selectors:
+
+    <StackPanel>
+      <StackPanel.Styles>
+        <Style Selector="Button.blue">
+          <Setter Property="Button.Background" Value="Blue"/>
+        </Style>
+      </StackPanel.Styles>
+      <Button Classes="blue">I will have a blue background.</Button>
+      <Button>I will not.</Button>
+    </StackPanel>
+
+## Pseudoclasses
+
+Also as in CSS, controls can have pseudoclasses; these are classes that are
+defined by the control itself rather than by the user. Pseudoclasses start
+with a `:` character.
+
+One example of a pseudoclass is the `:pointerover`
+pseudoclass (`:hover` in CSS - we may change to that in future).
+
+Pseudoclasses provide the functionality of `Triggers` in WPF and
+`VisualStateManager` in UWP:
+
+    <StackPanel>
+      <StackPanel.Styles>
+        <Style Selector="Button:pointerover">
+          <Setter Property="Button.Foreground" Value="Red"/>
+        </Style>
+      </StackPanel.Styles>
+      <Button>I will have red text when hovered.</Button>
+    </StackPanel>
+
+Other pseudoclasses include `:focus`, `:disabled`, `:pressed` for buttons,
+`:checked` for checkboxes etc.
+
+## Named Controls
+
+Named controls can be selected using `#` as in CSS, e.g. `Button#Name`.
+
+## Children
+
+As with CSS, you can select children and descendants:
+
+- `StackPanel < Button#Foo` selects a `Button` named `"Foo"` that is the child
+  of a `StackPanel`.
+- `StackPanel Button.foo` selects all `Button`s with the `foo` class that are
+  descendants of a `StackPanel`.
+
+## Templates
+
+You can select controls in the template of a lookless control by using the
+`/template/` selector, so `Button /template/ Border#outline` selects `Border`
+controls named `"outline"` in the template of a `Button`.