feat(themes): change placeholder and helper text colors dark themes

[aledavila]

Closes #9860
Closes #9832

New colors for $text-placeholder and $text-helper in themes g90 and g100
The decision was made not to add them to the layer sets and just change the global token color

Changelog

Changed

g90

• changed $text-placeholder from Gray 60 to Gray 50
• changed $text-helper from Gray 50 to Gray 30

g100

• changed $text-helper from Gray 50 to Gray 40

[joshblack]

@aagonzales @aledavila are there any component implications for this change or does the value change here cover everything? 👀

[aagonzales]

It should cover everything. To my knowledge placeholder text is only used in inputs. Placeholder text also never passes color contrast so the fact that it does vary across layers is a moot point from the start.

[aledavila]

@joshblack should cover everything since like anna said we only need to make them a bit more visible so they don't match the input color. Helper text as well

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[Eugeno]

You've made placeholder visible, but what about textDisabled? =)
Same story: gray100 theme, field-03, gray60 text on the gray60 background.

[aagonzales]

@Eugeno I address that in this issue here: #10054