It appears that after the latest AGO update, the separator color within dashboards does not work as it previously did. In the screenshot below, I have one object in a list widget. Previously, I would make the separator the same color as the background so it didn't show. Now, after the update, it reverts back to the default. And even if I use the advanced formatting, nothing appears to overwrite the default value.
Also, if I remove advanced formatting and try to manually type in a value using the color wheel, that doesn't seem to hold either, using the hex code or RGB values. Only the color wheel seems to work.
