Improve caroussel button definition

[Samuel-Therrien-Beslogic]

• The project passes automated tests (build, linting, etc.).
• You updated the project's documentation with new changes.
• You've linked any issue this PR closes
• You reviewed your own PR and made sure there's no test/debug code or any obvious mistakes.

Make sure that the code wasn't copied from elsewhere (check one):

• This is your own original code
• You have made sure that we have permission to use the copied code and that we follow its licensing

A nudge towards #280

There's a visual change to the close button:
Before

After

[andy2great]

LGTM