Add support for other themes

[jtpio]

Transferring jtpio/jupyterlab-theme-toggle#3 here in this repo.

For now the theme toggle extension only support the dark and light themes:

jupyterlab-topbar/packages/theme-toggler-extension/src/index.tsx

Lines 90 to 93 in 4eb59bc

	const themes = [
	'JupyterLab Light', // Light Theme goes first
	'JupyterLab Dark',
	];

However this could be made configurable via the settings system.

[mahendrapaipuri]

How do you envision this? Changing to sort of dropdown component and letting users to configure more than two themes or still use toggler component where users can configure two themes at a given time?

[jtpio]

Since there is already the possibility to choose from a list of themes via the Settings menu, maybe limiting the toggle to 2 options would make sense?

End users would go to the settings editor to enter the names of the two themes they would like to use.