How to write file to assets

We have ```readFileAssets```, but no ```writeFileAssets``` how can I write files to android assets folder
or how the relative path between assets folder and DocumentDirectory?

Below is the problem I got in real word:

I have a index.html file in ```\android\app\src\main\assets``` for my webview component and in this html I include some CSS files like`````` if I put those CSS files under the ```\android\app\src\main\assets``` then it works fine but now some CSS files will be download from API.

I can save them to DocumentDirectory folder but can't save them to assets folder so I can't include those CSS file, anything will be help. Thank you.