Some things uncovered in #6961 that should be brought back to the Merge Conflicts dialog once #6961 is merged:

• add support for setting JSX for title on Dialog props #6032 means we should set Dialog.title to a JSX element (and update the CSS because of the DOM changes)

And also for the Merge Branch selector:

• these colours should be replaced with variables defined in variables.scss values (or add new values) to ensure they can be styled correctly with different themes

• the arrow octicon should have an explicit height and width if set in CSS - more context here [rebase] menu item and dialog to start rebase operation #6961 (comment)