If we are talking about this theme, it seems that it was last updated 2 years ago.
In that time frame, much has changed in the way GTK3 draws it's themes. These dashed lines seem to be a side effect of out-of-date CSS.
The bad news is - the theme seems to be out of date, and will not render correctly in recent GTK.
The good news is, you can look at how other themes have handled this (example with vertex theme) and try to fix it yourself. You can find the reference guide here.
You just need to handle the GTK3 related stuff, which should not be that much code.
I know it's probably not the answer you were looking for, but AFAIK it's the only change you have to get it to work correctly.
The same is valid if the problem is in the One more flat theme.
Good luck trying to modernize the theme. I wouldn't say it's the easiest task in the world, but it shouldn't be the toughest one either. Especially if you are already familiar with GTK theme file locations. Bonus points if you are already familiar with CSS.