Audit the components color and use only colors from the palette

[afercia]

We shouldn't ever use hardcoded colors, and always use color variables from the palette as that's the only way to keep colors under control in the long term. For example:

composites/Plugin/SnippetEditor/components/Shared.js > InputContainer uses:

border: 1px solid ${ ( props ) => props.isActive ? "#5b9dd9" : "#ddd" };

===

a few default props in Button.js use "#000"

===

a11y/Styles.js
backgroundColor: "#eeeeee "

===

also hardcoded colors in the styled components are not ideal, for example:

SnippetPreview DesktopContainer:
background-color: white;

SnippetPreview MobileContainer:
border-bottom: 1px hidden #fff

===

About rgba colors, we have a rgba() utility function that should be used instead of hardcoded rgba colors.

===

I guess the only exception is about the colors used for the snippet preview, which mimic the Google ones.

===

If new colors are needed, these should be discussed with the design team for inclusion in the palette.