"Merged" with Custom CSS Tweaks

It turns out that we nearly wrote the same code. However, you did yours from a variable directory, where I did mine from a static directory.

This is basically your code with added tweaks:
- The preferences for Justify and Linespacing is disabled when Custom CSS is in use. This is to ward away potential conflicts in code.

- The function sending the CSS to the HTML header merges all three custom CSS, justify, linespacing functions into 1

- Merging functions allowed the reader to revert to default CSS when custom CSS can't be found. (Toast alert to display error).

- CSS Layout related items are now in their own preference sheet.