Sure, you can add a single ‘plain’ horizontal line for < hr > (I’ve added a space either side of hr here purely for display purposes, so normally, there should be no spaces).
I’ve tested the following fix on several current GetPublii.com themes. It seems the most recent GetPublii.com themes have the necessary CSS code built in.
After you’ve added the default < hr > as shown in your first screenshot above, assuming you’re using WYSIWYG view, choose the ‘<> HTML’ button on the editor bar to view the code.
Find your < hr > then change:
Once you’ve saved your changes on the page, when you preview the page or the site, the page on which you’ve made the change should display your ‘plain’ horizontal line.