Login screen: Replace hardcoded CSS colors with Color Studio as per design guidelines

[codebykat]

The auth screen colors were mostly defined as pure CSS. This PR replaces them with Color Studio colors as per the design guidelines.

The changes are most drastic in dark mode.

Screenshots

Light Mode Before:

Light Mode After:

Dark Mode Before:

Dark Mode After:

Test

Build the Electron app and view its login/sign up screen.

Release

RELEASE-NOTES.txt was updated in 7b57bc6 with a link to this PR.

[codebykat]

@SylvesterWilmott I am troubled by how different the shade of blue originally was for the login with WPCOM button -- is it supposed to be different than our "brand blue"? Pls advise

[belcherj]

Well, the colors being hardcoded is totally my fault. I wanted to start using the new colors but didn't want to change all the colors on the site.

[SylvesterWilmott]

It looks great @codebykat thanks!

I am troubled by how different the shade of blue originally was for the login with WPCOM button -- is it supposed to be different than our "brand blue"? Pls advise

We should use the WP brand blue for this button specifically WordPressBlue50

Otherwise, just one minor change:

• Input placeholder text (Dark): gray20

[belcherj]

Ship it!

[codebykat]

Fixed up the WPCC button background and the input placeholder colors for dark mode, merged 👍