Support for custom CSS

[fresh2dev]

Termynal can be customized quite heavily with CSS alone.

For example, I replace the MacOS-like modal buttons with standard '- ⛶ X' text.

And I replace the title "bash", with plain "shell".

And if the language is "powershell", I set the prefix to 'PS >'.

Similarly, for "python", the prefix is '>>>'.

Rather than baking all this into this tool, if this tool could allow me to point to a custom CSS file, it would simplify the Termynal-related markdown I maintain today.