[DDW-453] Earned delegation rewards "export CSV" button update styles

[topseniors]

This PR updated header section styles of delegation rewards screen so that header section gets sticky when scrolling content section
Jira Ticket

Todos

• Update header section styles of delegation rewards screen so that header section gets sticky when scrolling content section

Testing Checklist

• Slack QA thread

Test Cases

Scenario 1: “Earn delegation rewards” and “Export CSV” button section remains sticky
Given I am on Staking -> Reward
And I am at the Reward Table
And the Reward Table has data displayed
And the Reward Table is scrollable
When I scroll the Reward Table
Then the Reward Table will scroll
And the Reward Table header will scroll
But “Earn delegation rewards” and “Export CSV” button section remains sticky

App

• Go to Staking => Rewards and make sure header section gets sticky when scrolling content section

Screenshots

English

Japanese

Review Checklist

Basics

• PR has been assigned and has appropriate labels (feature/bug/chore, release-x.x.x)
• PR is updated to the most recent version of the target branch (and there are no conflicts)
• PR has a good description that summarizes all changes and shows some screenshots or animated GIFs of important UI changes
• CHANGELOG entry has been added to the top of the appropriate section (Features, Fixes, Chores) and is linked to the correct PR on GitHub
• Automated tests: All acceptance and unit tests are passing (yarn test)
• Manual tests (minimum tests should cover newly added feature/fix): App works correctly in development build (yarn dev)
• Manual tests (minimum tests should cover newly added feature/fix): App works correctly in production build (yarn package / CI builds)
• There are no flow errors or warnings (yarn flow:test)
• There are no lint errors or warnings (yarn lint)
• There are no prettier errors or warnings (yarn prettier:check)
• There are no missing translations (running yarn manage:translations produces no changes)
• Text changes are proofread and approved (Jane Wild / Amy Reeve)
• Japanese text changes are proofread and approved (Junko Oda)
• UI changes look good in all themes (Alexander Rukin)
• Storybook works and no stories are broken (yarn storybook)
• In case of dependency changes yarn.lock file is updated

Code Quality

• Important parts of the code are properly commented and documented
• Code is properly typed with flow
• React components are split-up enough to avoid unnecessary re-renderings
• Any code that only works in main process is neatly separated from components

Testing

• New feature/change is covered by acceptance tests
• New feature/change is manually tested and approved by QA team
• All existing acceptance tests are still up-to-date
• New feature/change is covered by Daedalus Testing scenario
• All existing Daedalus Testing scenarios are still up-to-date

After Review

• Merge the PR
• Delete the source branch
• Move the ticket to done column on the YouTrack board
• Update Slack QA thread by marking it with a green checkmark

[miorsufianiohk]

Hi @yakovkaravelov , @alexander-rukin found issues with scrolling start/end position on build 17528. See video here

found issues with scrolling

[topseniors]

@miorsufianiohk @tomislavhoracek @alexander-rukin Please review/test. Thanks.

[tomislavhoracek]

✅

[alexander-rukin]

Looks ok

[miorsufianiohk]

LGTM 👍 . Tested on 17541