Use Greenfield-ish style for buttons

[ryonakano]

Prerequisites

• I have searched open and closed issues for duplicates.

Feature

Is your feature request related to a problem? Please describe.

The OS will use the new stylesheet called Greenfield in the next version. So, it would be great to update the style for buttons on the website too to a similar style with it for consistency.

Describe the solution you'd like
Make the style for buttons on the website Greenfield-ish.

Existing work
I guess we can refer to how we define the button style in Greenfield.

Describe alternatives you've considered
N/A

Additional context
N/A

[cassidyjames]

We are likely going to rework the homepage (and maybe a lot of the site? We'll see) for the OS 6 release. So this should be covered by that.