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

Imagine I want to visit webmaster.stackexchange.com (or an other website) everyday but I want to change the blue color of header (change it to red for instance).

Is it possible to force personal css styles when I visit a website? If so, how can I do it? Maybe a web browser tip or a plugin? (I use Chrome)

I would like an automatic solution, change css with web browser console (F12 on Chrome) for each page I visit is not interesting.

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).

Greasemonkey ... a google search pointed me to Tampermonkey (https://chrome.google.com/webstore/detail/tampermonkey/dhdgffkkebhmkfjojejmpbldmpobfkfo?hl=en)