Fix welcome page design on CE

What does this MR do?

While trying out CE, I've noticed that the design of the welcome page is off. This MR fixes the problem.

The width for the welcome page card wasn't set since the style definition was in EE. Since the welcome page layout is also used for other steps, the margin: 0 set on the container div made the card not aligned in the middle of the page.

How to try it out

1. gdk stop (if running)
2. FOSS_ONLY=1 gdk start
3. Signup as a new user. After you click on "Register", you will be redirected to the welcome page that you see in the screenshot below.

Screenshots (strongly suggested)

Before	After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team