Footnotes in dark theme are invisible

In the dark theme, footnotes render black against the black background, which makes them invisible.
Fix `boostlook.css`. It probably requires adding

```
.boostlook #footnotes,
.boostlook #footnotes .footnote,
.boostlook .footnotes,
.boostlook .footnotes .footnote {
color: var(--text-main-text-body-primary);
}
```

But I have no means of verifying the result.

Open boostlook.css and inspect the existing dark-theme rules for #footnotes and .footnotes. Apply the requested footnote color change, then verify in a dark-theme rendering that footnote text is visible against the background.

Written by the indexing model from the issue text.