I’m currently experiencing this issue (refer to picture above). It started after I upgraded to Mojave and it appears on both dark and light mode
Honestly that looks like a hardware issue. The fact that it happened after you upgraded to Mojave is merely a coincidence. You can try a few things to see if it persists.
- Boot into safe mode by holding down the Shift key while booting up your Mac.
- Boot into Recovery Mode by holding down Command and R while booting up your Mac.
Safe Mode will turn off any software that has been added to your Mac and leave only what is necessary and from Apple. On the off chance that some software is corrupting the display.
Recovery Mode will boot you from the recovery partition, completely eliminating macOS as installed on your Mac as the culprit. So if it still looks the same it will need to visit a repair shop.
Likely issues are a bad video card (motherboard), bad screen, or bad cables connecting the two. Take it to an Apple store or a trusted Apple Authorized service center if it is out of warranty or you are not near an Apple store.