Fix alert button styling so that they don't show up white

Clement Ho requested to merge fix-alert-btn into master

What does this MR do?

Fix alert button styling so that they don't show up white

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Before After
Screen_Shot_2018-06-25_at_2.48.16_PM Screen_Shot_2018-06-25_at_2.50.45_PM

Does this MR meet the acceptance criteria?

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/48407

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