fix(border-subtle): bump value in dark themes

[tw15egan]

Closes #14597

@janhassel pointed out that our dark theme $border-subtle values were lagging behind the corresponding $layer values.

Changelog

Changed

• Bumped up the values of border-subtle-01 and border-subtle-02 in g90 and g100 so that they are visible in Layer Two and Layer Three. It seems a (possible) cause of this is that the Theme component assumes (and sets) a layer-one class by default, and not the background. We possibly need to look into setting a background class by default instead and creating new -00 layer tokens. See here and here. We also emit layer-01 at root, instead of a possible layer-00, but this is the easiest fix and doesn't open a can of worms.

Testing / Reviewing

Go to Layer --> Test Story to see value changes. Check other components in g90 and g100 to ensure the subtle borders look correct

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[tw15egan]

@guidari and I bumped up values for the border-subtle-01 and border-subtle-02 tokens so that they were visible (see here for example). These changes would only affect a component in g90 or g100 on Layer Two or Layer Three. Let me know if these visual changes are acceptable -- You can view any With Layer story to see/inspect the changes or view the Percy link

[alisonjoseph]

LGTM! 👍

[tay1orjones]

Just need to remove the test/demo stories before we can merge.