I have a css file to change field color in a form but I have no idea which bundle should contain it.
You can read in the Bundles documentation that the web.assets_backend
bundle contain the code specific to the web client (notably the web client/action manager/views):
web.assets_backend:
this bundle contains the code specific to the web client (notably the web client/action manager/views)
Example (taken from the web
module):
'assets': {
'web.assets_backend': [
'web/static/src/legacy/scss/fields.scss',
],
}