Fix job page in dark mode

What does this MR do?

Update SCSS variables to use the dark-mode ready CSS vars.

Mainly to fix an issue with retried jobs in the sidebar, as seen on https://gitlab.com/gitlab-com/security-awards/-/jobs/1136463534

Updated other broken dark mode colors in the same file while I was on the area. Few tiny changes in light mode, they are imperceptible unless viewed side-by-side.

Screenshots (strongly suggested)

before	after

Does this MR meet the acceptance criteria?

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