Mostly based on showingfupforracialjustice page
White Supremacy Culture is a bunch of characteristics which are usually found in groups, companies and organisations dominated by whiteness. They are established de-facto “best practice” standards. Lots of these can be found in (but are not! limited to) typical capitalistic scenarios.
These characteristics are as stated below. For details and how to approach them, see the link above.
They are harmful because they supress other approaches and thus promote white supremacy. They are not limited to white people and the harm they are doing is not limited to people of color.
The linked article states multiple antidotes for each of the characteristics. Lots of these tactics can be boiled down to a culture in which: