Not sure if I understand you issue, but it seems to me that your a defining a class in a web block? If that is the case you must read CSS Specificity here: https://success.outsystems.com/Documentation/11/Developing_an_Application/Design_UI/Look_and_Feel/Cascading_Style_Sheets_(CSS)#css-specificity
This will give you a better idea which class be applied depending where did you define them.
Otherwise, is it possible you create oml that will demonstrate the issue.
Hope this helps in any way.
Regards,
Arnold