Dark theme support

[CharlBest]

Duplicate of #40 but with correct formatting and linting

As I mentioned. I don't mind if this doesn't go in, so feel free to close/disregard it if you are not interested 😀

Basic theme swap inspired by Stripe Dashboard. Initially, it was very minimal with just the svelte component. The problem is you lose what you choose if you go to dark mode. Maybe not a problem but I added a cookie to save your preference.

Default:

Dark:

[NDruce]

Performing replacement in whole HTML code is excessive overhead and bad way. Better make it client side only, don't store/handle anything that you can on server - it's excessive in this case too. Use localStorage instead of Cookies (they are also increasing amount of data transferred in each request) and handle this logic only on client side, server side is unnecessary for this feature.

Also Cookies code crashing app server because "you set Cookie after content was started" (smth like that error was in crash reason message)

UPD: I recoded

PR: #59