Add theme (light / dark mode) switcher to user dropdown

[eddybrando]

[JRussellHuffman]

The design for this is included in the user profile figma file, located here: https://www.figma.com/file/SOnOgCGQIWx4oAlSFhIsIX/1972-User-account?node-id=615%3A4580

The design includes a toggle for light, dark, and system modes.

[vabarbosa]

the initial work regarding light/dark mode can be viewed here

• https://github.com/qiskit-community/platypus/pull/143

currently, it is set based on user's system preferences

[eddybrando]

We'll wait until we have the new "user profile" button in the main bar to start implementing this (https://github.com/Qiskit/platypus/issues/850).

[Eric-Arellano]

When adding dark mode support, see https://github.com/Qiskit/qiskit_sphinx_theme/issues/273 for how to add it to documentation too via the Furo Sphinx theme. We will want to have the toggle live in a generic part of qiskit.org, like the top nav bar. But we can leverage Furo's support for dark mode like it's Sass setup.

[y4izus]

We decided to not implement the dark mode, so I close this issue. If at some point we decide to do it, we can create a new issue