A11y: use black for schedule header to fix contrast

[hugovk]

For https://github.com/EuroPython/website/issues/682.

Before

Use primary colour (dark blue #0409e7) for schedule table headers. The ratio on the advanced #D34847 background is 2.17:1: https://webaim.org/resources/contrastchecker/?fcolor=000000&bcolor=D34847

After

Use black as the foreground colour. The ratio on the advanced #D34847 background is 4.78:1 https://webaim.org/resources/contrastchecker/?fcolor=FFFFFF&bcolor=FF0000

The other levels all remain above the WCAG AA min of 4.5:1.

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