Here are my icons and here is the Inspect Element for them:
I’ve tried all the CSS coding you listed, both trying “stroke” and “fill.” The “!important” thing didn’t work either. I have no idea what’s causing this.
I might have stumbled upon something, though. I noticed that even though I am using .SVG “images”, the Inspect Element is still showing them in an <img> tag VS the <svg class=" that yours shows. Is this a bug with Custom Header Icons?