Fix color mistakes in mastodon-light theme (#7626)

* Fix colors of mastodon-light theme

Fix colors of modals and focused toots in light theme
Fix colors of compose-form items and more
Fix colors of status__content__spoiler-link:hover and $valid-value-color
Change success green color in light theme

* Fix some sass codes

* Add !default for explicit color valiables in default theme

for overwriting colors easier in the other themes