Style issues for `FullscreenImageCarousel` in mobile and dark mode

[qroll]

Component
FullscreenImageCarousel

Describe the bug

There are a couple of style issues for the carousel in mobile and dark mode

1. In small screens on landscape mobile, the buttons on the right will overlap or are too close together. The component should apply the mobile variant styling in mobile landscape as well (similar to ESignature component)
2. The bottom section should retain the background colour as #282828 in dark mode
3. Remove icon button's box shadow in dark mode

To Reproduce
Steps to reproduce the behavior:

1. Go to the default story https://designsystem.life.gov.sg/react/index.html?path=/story/modules-fullscreenimagecarousel--default
2. Switch the viewport to a mobile viewport e.g. iPhone 5
3. Rotate the viewport to landscape
4. See error

Screenshots

iPhone 5

iPhone 6