Contact Form: Update CSS stylesheet

[thomasguillot]

Changes proposed in this Pull Request:

• Display input elements full-width on small screen and when using the column block
• On large screens, set input elements to 50% and texture to 100%
• Replace px with em
• Add style for Checkbox Multiple
• Set font-weight of Radio/Checkbox labels to normal so we can differentiate with actual labels

Testing instructions:

• Add a contact form to a page, make sure you use all available elements
• Check the front-end, how does it look?
• Resize the screen, is it full-width?
• Add a contact form to a Column block, is it full-width when previewing the page?

Before:

After:

Proposed changelog entry for your changes:

• Contact Form: Update CSS stylesheet

[matticbot]

Caution: This PR has changes that must be merged to WordPress.com
Hello thomasguillot! These changes need to be synced to WordPress.com - If you 're an a11n, please commandeer, review, and approve D27670-code before merging this PR. Thank you!

[jetpackbot]

	Warnings
⚠️	The PR is missing at least one [Status] label. Suggestions: [Status] In Progress, [Status] Needs Review

This is an automated check which relies on PULL_REQUEST_TEMPLATE. We encourage you to follow that template as it helps Jetpack maintainers do their job. If you think 'Testing instructions' or 'Proposed changelog entry' are not needed for your PR - please explain why you think so. Thanks for cooperation 🤖

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[jeherve]

It seems to work well for me. 👍

[iamtakashi]

Design wise 👍for me. This is a nice design upgrade, thank you, @thomasguillot !

[scottsweb]

Add a contact form to a page, make sure you use all available elements

👍

Check the front-end, how does it look?

Good, seems clean.

Resize the screen, is it full-width?

Yes

Add a contact form to a Column block, is it full-width when previewing the page?

Seems to be full width within the column:

So, all good I think. 👍