Where should I place my css file in odoo15, web.assets_backend or web.assets_frontend?

Question

I have a css file to change field color in a form but I have no idea which bundle should contain it.

Answer 1

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',
    ],
 }

Answer 2

In odoo 15 CSS, SCSS, and JS files path is added in manifest file check website module manifest file for reference, Odoo documentation for different bundle selection.

It is best to check which CSS file is affecting the view you want to enhance and check its file path and add your CSS accordingly, so the question about assets backend or frontend depends on where you want to enhance. check the documentation that might also give you a better idea