Are you using a Leaflet plugin to add those buttons? I've been doing a bit of work with Leaflet (just in JS, not using the R wrapper), and most of the buttons for the default controls don't work this way. For example, the zoom control just uses
- characters pulled from the DOM element contents, and the layer control uses an icon that's stored in a separate file:
If that's just the way the plugin does it, it might take some more involved CSS to resize the buttons neatly. I'd try
background-position. Still, that's tough
If you're writing CSS to style the controls, you could also just swap in your own background images too! Then you don't need to piss around with trying to resize the graphics to match your buttons (especially if you're using SVG icons).