Add theme support

[GeorgeSG]

👋 Hi @oppiliappan,

Love the project you've built and I'm happily running it in my homelab! I'd be happy to contribute some features / improvements if you feel they're a good direction for lurker.

Apologies for the consecutive PR. I got a bit too eager to update my master 😅

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This adds support for custom theming.

In order to enable a theme, you must do the following:

1. Set the LURKER_THEME environment variable to the theme name
2. Have a <theme-name>.css file present in src/public.

Example

• LURKER_THEME=catppuccin
• mount catppuccin.css in /src/public/
• catppuccin.css content:

    @media (prefers-color-scheme: dark) {
    	:root {
    		--bg-color: #1e1e2e;
    		--bg-color-muted: #313244;
    		--text-color: #cdd6f4;
    		--text-color-muted: #bac2de;
    		--blockquote-color: #a6e3a1;
    		--sticky-color: #cba6f7;
    		--gilded: #f9e2af;
    		--link-color: #94e2d5;
    		--link-visited-color: #bac2de;
    		--accent: var(--link-color);
    		--error-text-color: #eba0ac;
    	}
    }

[oppiliappan]

this is very cool, thanks, will take this for a spin later today.