Opt out of !important CSS rules generated from theme.json

[MarieComet]

Proposed solution for #37590

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[aristath]

At this stage, we want to avoid adding filters to the theme.json parser and global styles. That's not to say that filters will not be added, but we need to come up with a viable strategy. Perhaps there will be a single filter for the generated CSS, in which case that can be used to remove the !important strings all at once.
Let's not rush this until we have a better idea of how we'll be able to hook in those styles. Adding a bunch of filters for each scenario would not be an optimal solution and we need to find a way to make these hookable but abstract enough to be useful in more than one scenarios.

[oandregal]

In addition to what Ari shared, there's already ways to hook into these styles. I'll share more in the original issue.