Persona/lightweight themes must currently be added by the user. When added, the persona/lightweight theme is recorded in "about:config" as a "user set" preference ("lightweightThemes.usedThemes"). Until the user adds a persona/lightweight theme to Pale Moon, the "lightweightThemes.usedThemes" preference is empty and hidden. Instead of "lightweightThemes.usedThemes" being empty by default, a few persona/lightweight themes could be easily preloaded in "lightweightThemes.usedThemes", as the default value of the "lightweightThemes.usedThemes" preference. That is, change the default value of "lightweightThemes.usedThemes" from:
These light themes require no downloads or external images. These light themes all use one image file ("slider.gif"), a 2px file already included in Pale Moon chrome. As a test of concept, I've listed eight persona/light themes. But, of course, the default value of the "lightweightThemes.usedThemes" preference could contain fewer or more light themes, or entirely different light themes from the above. And, more complex persona/light themes could be added to the defaults, by adding needed light theme images to Pale Moon chrome.
The latest versions of Firefox no longer support complete themes. And, Firefox calls persona/light themes simply "Themes". In contrast, the Pale Moon add-on site calls its healthy number of complete themes "Themes", whereas persona/lightweight themes are no where to be found or referenced. These differences have to be confusing to Pale Moon users migrating from Firefox or other browsers. Including a few light themes in the default value of the "lightweightThemes.usedThemes" preference-- such inclusion should partially mitigate this confusion.