Can no longer edit CSS

Theme: Nucleare

Whenever I try and use /wp-admin/themes.php?page=editcss or the cut-down version in the customize section, the page fails to load properly and I can’t edit the CSS. It crashed this morning as I was editing CSS, and I haven’t been able to make changes since. I’ve tried to edit the CSS using different browsers and even different PCs to no avail. The issue must be at WordPress’s end.