hydrogen-web/doc/SKINNING.md

Replacing javascript files

Any source file can be replaced at build time by mapping the path in a JSON file passed in to the build command, e.g. yarn build --override-imports customizations.json. The file should be written like so:

{
    "src/platform/web/ui/session/room/timeline/TextMessageView.js": "src/platform/web/ui/session/room/timeline/MyTextMessageView.js"
}

The paths are relative to the location of the mapping file, but the mapping file should be in a parent directory of the files you want to replace.

You should see a "replacing x with y" line (twice actually, for the normal and legacy build).

Injecting CSS

You can override the location of the main css file with the --override-css <file> option to the build script. The default is src/platform/web/ui/css/main.css, which you probably want to import from your custom css file like so:

@import url('src/platform/web/ui/css/main.css');

/* additions */