Some child theme CSS is not showing up during private browsing

I’m having a weird problem that I haven’t run into before with any other themes. I created a child theme for Panel that I’m attempting to use, and made some CSS modification – light background for the header with dark text. Everything seems to work fine and the changes show up as expected on both Chrome and Firefox… Unless I’m using incognito mode in Chrome, or private browsing in Firefox, in which case the light header background does show up, but not the changes to the header font color… So I end up with white on white text.

It doesn’t seem to make any difference whether the browser cache is cleared, the user is logged in, etc. I have NO clue what’s going on… Can anyone familiar with the programming behind this theme or who’s run into any similar issues help? Thanks!