How can I force my css styles when I visit a website?

The general concept you're after is "user stylesheets."
Stylish (Chrome / Firefox) gives you an easy way to manage per-site styles and toggle them on/off as needed.

In 2017 Stylish became very heavy and full of bloat features so I decided to move away from it.

I have created an alternative - Styler extension - it's lightweight and has minimal style but powerful. You can add custom CSS and JavaScript/jQuery code.

Styler Extension for Google Chrome

Styler Extension for Firefox

Source code

Why I love it and use it on daily basis - mostly because of autosaving and auto-syncing between all Google Chrome browsers (on Mac, Win and Linux).