Fix link readability: replace yellow link color with underlined body text

[Copilot]

Links inherited the UU primary color (#FFCD00) — bright yellow on white — making them nearly unreadable. This aligns link styling with standard UU branding: same color as body text, underlined.

Changes

• docs/stylesheets/extra.css — adds two overrides scoped to both default and slate color schemes:
  • Overrides --md-typeset-a-color to use --md-default-fg-color (dark body text) instead of the yellow primary
  • Applies text-decoration: underline to anchor elements

[data-md-color-scheme="default"] .md-typeset,
[data-md-color-scheme="slate"] .md-typeset {
    --md-typeset-a-color: var(--md-default-fg-color);

    & a {
        text-decoration: underline;
    }
}

[Rene-Universiteit-Utrecht]

/lgtm