1. Master Pages & Themes

2. .master May have several in a site Add New Item / Master Page
.master file is template
.asp is content
MasterPageFile="~/myMaster.master"

3. Where
<asp:contentplaceholder id="ContentPlaceholder3" runat="server">
If you have content, it will be default content.
Be careful with relative URL's, which are relative to the master.

4. URL Alternatives
Relative
Absolute
Programatically Generated

5. Themes
Theme = "simple2"
In directive

6. Skins Make a folder App_Themes
Any folder in it contains a theme, by the name of the folder
Name your folder uniquely
.skin
Can only affect appearance
SkinID="myTextBox"

7. Skin is a default control, with appropriate style.
EnableTheming = "false"
You can do much of this in web.config
CSS is automatically applied if in the folder, even if more than one
Can apply Themes and Skins dynamically, in PreInit event.