Remove all instances of SCSS and replace with CSS

[karnikaavelumani]

What happened?

CSS is clear for beginners to use and learn. To make the code consistent, we should use the same type of style throughout the repo.

[JustinStitt]

SCSS is a superset of CSS. I believe beginners can choose to use CSS as they know and love.

Now, ofc, new devs still have to deal with reading that garbage in the source so I +1 this issue :)

[amyipdev]

I've also seen a general phasing out of SCSS, and given usage of Svelte, I think SCSS can and should be removed.