Adding new css variables for fw-file-uploader

[Tahseen-Ibrahim]

Checklist:

• [ ] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.
• [ ] My commits have standard messages as mentioned in Contributing Guidelines

How Has This Been Tested?

[arvindanta]

@Tahseen-Ibrahim You can close this PR, as we are going with css parts