Color contrast problems in Argo header

[andrewjbtw]

Describe the bug
There are a few places in the Argo header that seem to have color contrast issues.

In three cases, the text becomes darker after an interaction, making it hard to distinguish from the background.

1. Hover over "Argo" text

2. Expand "Register" dropdown

3. Expand "Impersonate" dropdown while impersonating (likely the same style issue as for "Register")

User Impact
This might be an accessibility issue? Even if the contrast meets WCAG guidelines, I find it hard to read.

To Reproduce
See screenshots above.

[astridu]

1. Hover on Argo: text should remain white, and underline should appear
   2 and 3. The text should remain white even when the dropdown is expanded.

[mjgiarlo]

@astridu 💬

1. Hover on Argo: text should remain white, and underline should appear

The component library prevents the underline from appearing due to sul-dlss/component-library#25. Not sure how to proceed. I'll assume we want what's in the component library and leave the hover out of this PR.

2 and 3. The text should remain white even when the dropdown is expanded.

OK, thanks!