White header in issues and PRs

[pdonias]

[schumar]

Additional info: This is a div.gh-header, which gets assigned background-color: #fff in discussion-timeline.scss

[schumar]

Fix: Add

.gh-header {
    background: var(--color-button-face) !important;
}

[pdonias]

It doesn't seem to fix pull requests' headers, though.