Popup's close button has too low contrast

[Malvoz]

The popup's close button fails all of the following WCAG success criteria:

• 1.4.3 Contrast (Minimum)
• 1.4.6 Contrast (Enhanced)
• 1.4.11 Non-text Contrast

(side note: PR #281 unintentionally fixed contrast issues of attribution links. 😋)