Go to a page like "Preferences / Colors and Fonts / Directory Opus Colors" (Prefs PAGE="opuscolors"), and select different list entries.
The checkmarks look like a child drew them: 
My DPI scaling is 125%.
This isn't the case with other checkboxes outside these color controls; they look clean:
The standard Windows checkmark doesn't handle non-integer scaling factors well. You can enable our own, non-child drawn checkmark via the color preferences (or switch to dark mode, which has all custom-rendered controls).
I just edited my post to show that other checkboxes look clean.
Other checkboxes may be sized differently. Jon gave you the solution above. If you tell Opus to render the checkboxes itself, instead of Windows rendering them, they scale properly.
Turn on Preferences / Colors and Fonts / Windows Colors / [Visual Styles] Checkboxes and radios for that.
