updated: removed gradient from onboarding buttons

[hnrykm]

Fixes #5168

• Added primaryInverted and primaryInvertedHover to design system.
• Changed primary button background with a gradient to inverted-flat for both light and dark themes.
• Hover added to go lighter (consistent with tertiary color of +5 step on GRAY_SCALE).
• Font color changed from primary to inverted.
• Modified button border from light to strong.

Two components are still utilizing the button with gradient background - email and chrome extension. Figma design guidelines show them to be inverted and flat (not gradient).

• Should I change those as well?
• Should the gradient style be removed altogether after this has been completed?

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[hnrykm]

My apologies. I'm an overeager new developer. I saw an issue that I thought I could take on and gave it a try even though it was already assigned to someone. I tried to save it as a draft until it was appropriate for me to submit a pull request. It doesn't look like it works that way. I don't mean to step on anyone's toes or bypass your guidelines.

[hnrykm]

Thank you!