MacOS – make dictionary result white text in spotlight in mojave dark theme

Try toggling it again; it should relaunch Finder with a fully dark theme, including dark backgrounds & white text...

Assuming I understand the question, the first thing I'd do is change your accessibility display settings. More specifically:

1. Go to Apple > System Preferences
2. Click on the Accessibility preference pane
3. Within the left-hand pane click the Display icon (it's usually the 4th one down)
4. Now to the right you'll see a number of checkboxes and sliders relating to accessibility display settings
5. Have a play with these to see if they improve the readability of light-grey text