Created attachment 9016 Screenshot of the application showing the problem When searching the word 'is' in the spellchecker it shows as following: '"is" is spelled correctly (en_US).' However the color of "is" is white, while the second 'is' is colored green.
Andre Miranda referenced this bugreport in commit 1606807a9fb9edd4d71e43a31d2909f976c31db3 Show correct colors (Bug #15946) https://git.xfce.org/apps/xfce4-dict/commit?id=1606807a9fb9edd4d71e43a31d2909f976c31db3
Colors were a bit off, the commit above fixes this problem. If the defaults colors don't play well with dark themes, you can still change them under Preferences -> General.