Audit hardcoded colors and replace them with variables

[afercia]

Noticed while looking at #703

The only way to to keep the colors palette under control in the long term, is to use only and exclusively colors variables from the palette. That's the reason why a style guide with a defined colors palette has been created.

However, over time, some colors have been hardcoded in the codebase, for example

yoast-components/composites/Plugin/SnippetEditor/components/Shared.js

Lines 57 to 58 in a3f623c

	background-color: #fff;
	color: #32373c;

where the white is a minor issue but #32373c isn't part of the Yoast palette and shouldn't be used. When there's the need for a new color, please discuss with the design team and carefully evaluate how often it will be actually used: if it's going to be used just once or twice, it's probably not really needed.

One more case of color that we've hardcoded is the #5b9dd9 blue from WordPress. as we're using it more and more, especially for the focus style, I'd propose to add it to the color palette. Alternatively, explore a way to have a sort of "JS style mixin" for the focus style that can be reused across the codebase.