Implement body gradient generation entirely in vanilla CSS

[cjmabry]

withtheranks-astro/src/styles/main.css

Line 204 in 5772e5e

/* TODO: Move second gradient color to css variables */

Currently there is a gradient in the background of the site that fades from the --main-bg-color to a lighter version of that color at the bottom of the page as the user scrolls, but the second color is currently hard-coded. Can we generate this second color purely with CSS, similar to SASS's lighten function, so that we could change it on the fly? https://sass-lang.com/documentation/modules/color/#lighten