Download stylish for your favorite browser from the link as show above in the post.
Once download, install it.
In your browser, open your favorite website (example: youtube.com)
Look at the bottom – right -corner of the screen for the icon of stylish (see the picture here)
Right click on the icon scroll all the way up for the option (“Find styles for this site”)
Select any style of your choice from this website (shown in figure) and install it . Now press refresh button in your browser.