Fix custom alert classes for dark theme

[SilasBerger]

In styles.scss

/* Custom Infima classes for Admonitions */
.alert {
  &.alert--definition {
    // TODO: Fix for dark theme.
    --ifm-alert-background-color: #faefe7;
    --ifm-alert-background-color-highlight: #eaded7;
    --ifm-alert-foreground-color: #003100FF;
    --ifm-alert-border-color: #988c8a;
  }
}