Where/how are you editing it? I’m assuming you’re editing the theme directly?
If you’re editing the theme and adding it to
common it won’t work, because it’s defined in
desktop (and you’d need to add it to the end of desktop). The C in CSS stands for Cascade, so you need to be sure that you’re making changes to
.d-header after where it exists in the theme.
I highly recommend creating a new theme component, adding your custom CSS there, and then add the component to the theme (rather than editing the theme directly). It would avoid this problem, and ensure your edits aren’t overwritten when the theme is updated.