Design Refresh - Update color tokens

Created by: umpox

Closes https://github.com/sourcegraph/sourcegraph/issues/19968

Designs: https://www.figma.com/file/NIsN34NH7lPu04olBzddTw/Design-Refresh-Systemization?node-id=908%3A7205

Description

This PR is the initial work to update our global styles to match the new colors in the design refresh.

• These color changes are scoped under theme-redesign, it should not have any impact on current production styles
• This PR does not aim to fully address our Bootstrap styles, this is a larger piece of work which https://github.com/sourcegraph/sourcegraph/issues/19969 will address.
• There is a limitation that our Sass variables (primarily used for Bootstrap) will not dynamically switch depending on our theme, as these have to be set at compile time. This will be addressed as part of https://github.com/sourcegraph/sourcegraph/issues/19969. See also https://github.com/sourcegraph/sourcegraph/pull/19926#discussion_r612511837.