USWDS - Side navigation: Fix current color link color.

[mejiaj]

Description

Resolves #4073.

The color on usa-current was being overriden due to color setting in over-specific selector a:not(.usa-button). This sets default color on a more specific a:not(.usa-button):not(.usa-current).

Additional information

There's a related feature request #3994 where we hope to revisit Side navigation overall. For example, it'd be better if we had a class like .usa-sidenav__link where we could differentiate sidenav links from other elements.

Test page created for this issue - preview link →

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