Make the styles not broken in 6.2

[jazzsequence]

The .welcome-panel class gets some new styling in 6.2 which makes the text virtually unreadable (black on dark blue). This PR flips the colors and adds some padding. Have not tested to see if these styles are enqueued globally but I don't think they are (looks like they're just here), but if this does affect things outside the importing page, a class could be added to the welcome-panel div wrapper to specify that it's a wp-import welcome-panel and then these CSS rules could be applied to wp-import.welcome-panel or just wp-import.