No class for images when working with a specific theme (no .field--name-field-) I have installed a new theme and when I go to DOM I can't see classes for the image fields; These are available for styling the page's image fields.

For example, if I write in the css file: .field--name-field-bestimageever img {display: none;} the image keep appearing.

This never happened to me with any other theme and I would thank an experienced themer for giving an explanation on how to add this, from which many other people could enjoy.
The most likely scenario is that the theme does not implement those field classes. Not all themes implement classes the same, but rather take a bottom up approach - the theme starts off very "clean" with only enough classes to support core behaviours etc, leaving it to the themer to add more classes as required. An example of this is the Stable base theme.