Fix darken than body bg color for form inputs

[dirkcuys]

https://github.com/p2pu/p2pu-theme/blob/2021-09-cr-dev/src/scss/_variables.scss#L430

[dirkcuys]

This is what a form on the default body background looks like with the current updates:

Even when in a container with a different background from the default, I'm not convinced this is clear that fields are enabled.

[dirkcuys]

Working on updates in Figma.

• Change input background to be white
• Add a border