Remove hardcoded colors from the bottom bar in favor of KDS theme

[Rajeev529]

Fixes #5672

This PR removes hardcoded background and border colors from the BottomBar component and replaces them with design system color tokens. the styles are applied using computed inline styles so the component supports dynamic theming. this change aligns the BottomBar with Kolibri’s design system guidelines and improves consistency across themes.

updated ui

inspect of the bottom bar

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[MisRob]

Thanks @Rajeev529!

Code changes make sense. I previewed locally and confirm that user experience is fine.

[Rajeev529]

@MisRob thankyou mam