It’s great to hear you’ve been enjoying Microthemer! You had the right idea with setting margin to auto. To center the logo, you need to apply this to the outermost div that you have limited the width of. You have set the width to 150px on the following selector: Div.col-md-3
And so you need to set margin-left and margin-right to auto there. Because otherwise you have a container div that is 150px wide aligned to the left. And setting margin to auto on child elements will only center them within the 150px wide box. Not the full width of the header area.