dark mode styling updates

[danditomaso]

Description

This PR relates to some further styling updates to help unify the overall colour scheme used on the Web Client.

Changes Made

• Issues with how buttons appeared in Dark mode were appearing around the site. It was important to address the primary button component and ensure for dark mode buttons appear correctly.
• For Buttons where they appeared on a dark background (the primary connection button), this is an edge case and was handled by directly updating the style for that one button.
• Usage of Gray and Slate were consolidated to use Slate, which provides a more uniform appearance across the site.

Testing Done

Visual testing

Screenshots (if applicable)

Before:

After: