Renames Dark theme to Midnight, and replaces Dark theme with a new Dark theme; Adds new Sepia and Forest inbuilt themes; Adds support for building custom themes directly using Pode.Web functions

[Badgerati]

Description of the Change

• Renames the current Dark theme to "Midnight", and replace the Dark theme with a proper dark theme
• Adds two new inbuilt themes: Sepia and Forest
• Adds support on Add-PodeWebCustomTheme, plus several helper function, to build your own themes directly in Pode.Web

To help with #449, all themed CSS files are now CSS variable files only, and nearly all theming is now done on the frontend not the backend.

Examples

Use the Dark theme as a base, and change the Page background to "darkred" and the font-family to "wingdings" (because why not!):

Use-PodeWebTemplates -Title 'Example' -Theme Custom
Add-PodeWebCustomTheme -Name 'Custom1' -Base Dark `
    -BackgroundColourConfig (New-PodeWebBackgroundColourConfig -Page 'darkred') `
    -FontFamily 'wingdings'