Low contrast ratio

[fflaten]

Certain colors should be changed to have a better contrast ratio (foreground vs background) for improved readability and accessibility. Some of these are shown by Lighthose in Chromium.

• [x] Dark mode: Active link
• [x] Both: Active section link
• [ ] Light mode: The pink and especially the turquoise color against the grey background used in the prism-theme (code blocks)
• [x] Light mode: The active page in the breadcrumbs and menu. Changing the background (--ifm-hover-overlay css-variable) slighty from 5% -> 4% alpha is an option.
• [x] Light mode: Placeholder-text in DocSearch. Could possibly ignore this.

[fflaten]

Related PR fixes everything above except prism-theme.