Description of problem:

There are a few UI issues about @backstage-community/plugin-rbac running on backstage-showcase.

1. No margin bottom of Create button
   
2. No margin top of `Download User List` button
   
3. Extra empty row on list view
   
4. No padding on action icons and icons aren't in same color
   
5. No margin of Cancel button on the create role page, and it should be a text button with no button background color
   
6. Delete Role Dialog looks off
   

Prerequisites (if any, like setup, operators/versions):

Steps to Reproduce

1. On latest backstage-showcase main branch, enable rbac plugin and start the app locally
2. Go to the rbac page

Actual results:

Refer attached screenshots

Expected results:

Figma role list page
Figma create role page

Reproducibility (Always/Intermittent/Only Once):

Always

Build Details:

Latest backstage-showcase main branch code with latest rbac plugin.

Additional info (Such as Logs, Screenshots, etc):