addtional notes in Themes.md

Technotes/Themes.md

@@ -1,7 +1,31 @@
 # Themes
 
-## Add Themes Directly to NetNewsWire
+## `.nnwtheme` Structure
-Theme developers: on iOS and macOS, themes can be opened directly in NetNewsWire using the below URL scheme:
+
+An `.nnwtheme` comprises of three files:
+- `Info.plist`
+- `template.html`
+- `stylesheet.css`
+
+### Info.plist
+The `Info.plist` requires the following keys/types:
+
+|Key|Type|Notes|
+|---|---|---|
+|`ThemeIdentifier`|`String`|Unique identifier for the theme, e.g. using reverse domain name.|
+|`Name`|`String`|Theme name|
+|`CreatorHomePage`|`String`||
+|`CreatorName`|`String`||
+|`Version`|`Integer`||
+
+### template.html
+This provides a starting point for editing the structure of the page. Theme variables are documented in the header.
+
+### stylesheet.css
+This provides a starting point for editing the style of the page. 
+
+## Add Themes Directly to NetNewsWire with URL Scheme
+On iOS and macOS, themes can be opened directly in NetNewsWire using the below URL scheme:
 
 `netnewswire://theme/add?url={url}`