fix: button links focus text visibility

[ptamarit]

:heart: Thank you for your contribution!

Description

Fixes #172

When the blue button-like link elements are focused, the rule .section-content-light-bg a:focus is setting the text to the same blue (#0377cd) as the background color (rule .goto-button a), making the text unreadable.

Adding the .goto-button a:focus to the existing rule already setting the text to white on hover and active fixes this problem.

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[ptamarit]

:heavy_check_mark: Just tested on https://inveniosoftware.org/ with the "See showcase gallery" and it's fixed.