fix(unity-bootstrap-theme): CSS fix for cards overlay

[davidornelas11]

Description

Cards with overlay gradient had wrong size.

Changed size to 100% to adjust for dynamic height

Links

• [Unity reference site](https://unity.web.asu.edu/@asu/unity-bootstrap-theme/index.html?path=/story/molecules-cards-examples--checkbox-stacked-card&args=header:false;footer:false;template:1
• JIRA ticket
• [Unity Design Kit](https://xd.adobe.com/view/56f6cb78-9af5-4b12-b4ce-ef319f71113f-03a5/screen/10657514-1129-405c-9726-23ac122ae4bf/specs/

Checklist

• [x] Unity project successfully builds from root yarn install & yarn build
• [x] Commits do not contain multiple scopes
• [x] Add/updated examples
• [x] No new console errors
• [x] Accessibility checks

Browsers

• [x] Chrome
• [x] Safari
• [x] Firefox
• [x] Edge