It doesn’t appear to be a button - it’s a link in a list item. I suggest selecting the list item from the layer tree and then apply your class to that. I must admit, selecting the borders you want the padding applied to can be a bit strange. Selecting one border seems to have a strange effect on the others, but eventually, the appropriate borders will be selected. Once selected, applying the padding/margin will work. I think this border selection problem may be an issue with your particular installation or file. If I add a button or link to a new project file, I can select the borders in the class manager without issue.