add light theme + theme switch

[FaberVitale]

Describe the problem

I want to be able to toggle between light and dark theme.

Describe the proposed solution

1. Add white theme

design a nice light theme then edit https://github.com/Roma-JS/roma-js-on-astro/blob/main/src/styles/main.scss#L144

2. Create theme switcher

add to the navbar a toggle button that on click switches between light and dark theme:

document.documentElement.classList.toggle(...)

3. Persist user preference

You need to check in a render blocking script that the default theme matches what the user has selected.

Alternatives considered

We could use a drop-down menu of we want to support more than 2 themes... but it does not seem that useful atm.

Importance

nice to have