I am implementing a secure browser based on Chromium which is used to browse sensitive web pages. I want to add watermarks to all web pages in my browser (without changing webpages at backend of course). Instead of writing chrome extension, I want to add this feature in Chromium source codes directly. However I am not familiar with Chromium codes, where can I start with?
If adding watermarks is something you want to do only for yourself via the browser, so only on YOUR browser you see these watermarks, you can use something like Tampermonkey (to add/edit/modify only in runtime) the webpages.
https://chrome.google.com/webstore/detail/tampermonkey/dhdgffkkebhmkfjojejmpbldmpobfkfo?hl=en
there are plenty of examples on how to add content to a webpage via this extension.