This seems like a potential cause of the problem. Our theme has a setting that stores content for our about page. I don’t think we had an SCSS compile error before updating to Discourse 3.4.0. Maybe something changed in sass compilation and now it’s more strict?
But the problem seems to occur when Discourse starts up. If I just edit the content in that theme setting there is no SCSS error logged. If I restart Discourse, the SCSS error occurs that color definitions CSS file becomes blank again.
Here is the code in the theme that uses that setting, in a hbs file. The content in the setting is just HTML.
<div class="about-description-content">
{{{theme-setting 'content_about_main'}}}
</div>