I am using Ubuntu 14.04 and Firefox quantum 59.0.1. But YouTube shows dark theme even the dark theme is turned off. I don't know why it is happening. Here is a screenshot.
How can I fix this isuue?
From your screenshot it seems you're using Enhancer for YouTube™ Firefox add-on which is capable of overriding your theme only for YouTube.
To fix this, open preferences for the add-on, scroll down to Theme section and select "Theme by default".