[UI] Minimalist backgrounds

[andersonjeccel]

Q	A
Bug fix? (use the a.b branch)	[ ]
New feature/enhancement? (use the a.x branch)	[ ]
Deprecations?	[ ]
BC breaks? (use the c.x branch)	[ ]
Automated tests included?	[ ]
Related user documentation PR URL	mautic/mautic-documentation#...
Related developer documentation PR URL	mautic/developer-documentation#...
Issue(s) addressed	Fixes #...

Description:

Yeah, this one may sound a bit boring.
Some changes were made so most of the main pages inherit the background color only removing bg classes.
It also makes separators invisible when you're creating a new contact or company: only space, no clutter.

Why?

• Because it becomes easier to work with colors across the entire ui
• Things get minimalist and clean, community seems to like it
• Minimalist interfaces are easier to build for new features thinking on your side
• It's based on feedback regarding lack of contrast, another PR were made to help and this one extends it and prepare for more

Before	After

Steps to test this PR:

1. Open this PR on Gitpod or pull down for testing locally (see docs on testing PRs here)
2. Open Contacts > Create new
3. Look at the fields and the page itself, there's a chance that you won't notice the difference

———

Pending tasks

• Revert hr style changes
• Remove hr tags from twig
• Include remaining tables on this PR, removing vertical lines and stripes
• Check again all pages to include instead of delivering only for main pages