Change the names of default themes

[akchurinda]

I have a suggestion of a purely cosmetic nature to rename the "documenter-dark" and "documenter-light" themes defined in the settings to simply "Dark" and "Light," respectively. I believe that the options "Dark" and "Light" would be more visually appealing for the end user.

[mortenpi]

Actually I would go one step further here and get rid of the notion of themes (from the reader's perspective). Just have "light mode" and "dark mode". And then package authors can override these if they wish, but can't add more themes.

Theoretically, right now, the idea is that you can have many themes, with two of them being marked the default light and dark theme. But I don't think anyone uses it that way, and so it just makes things complicated.

[Hetarth02]

@mortenpi With the introduction of new themes in #2496. Should we rename the current themes to just Dark and Light?

[vennelaeruvala]

I would like to work on this issue. Allow me to do it