feat(css): Update syntax of `mix-blend-mode`

[skyclouds2001]

Description

for mix-blend-mode, update syntax to support the plus-darker keyword value, which is supported since Safari 9

Motivation

Additional details

https://developer.mozilla.org/en-US/docs/Web/CSS/mix-blend-mode https://developer.mozilla.org/en-US/docs/Web/CSS/blend-mode https://drafts.fxtf.org/compositing/#propdef-mix-blend-mode https://drafts.fxtf.org/compositing/#ltblendmodegt

Related issues and pull requests