Adjust colors and button styles

[traceyljohnson]

Two updates here:

1. Tweak Tailwind hex values for Grayscale-500 → #374151 Grayscale-600 → #1F2A37 Grayscale-700 → #111928

2. Please adjust button components to reflect the new values seen here and here

Notes:

• Pay special close attention to the hover states used.
• Please ensure ALL buttons on the site are using one of these button variants. We will continue to ship updates and improvements to these buttons and want to make sure its as seamless as possible!

[gitstart-app[bot]]

Here is the GitStart Ticket for this issue: https://clients.gitstart.com/sourcegraph/3436/tickets/SGM-6995