Add light/dark mode option for users to switch between

[alexisreytech]

Look into adding a way for users to change whether they view the website in light mode or dark mode, in addition to system settings already in use.

[alexisreytech]

Here is a link to the Bootstrap Documentation for light and dark mode toggling.

[lsr-explore]

Questions

1. Should the header and footer also be updated? The footer background color is fixed to the dark blue
2. Where should the toggle be placed and what should it look like?
3. Should users have an option to go back to the system?

(also asked in this slack thread

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