Can I customize the appearance of a downloaded comic text box PNG, and what tools would I use?

Yes, you can customize a downloaded PNG using image editing software. Tools like Adobe Photoshop, GIMP, Krita, or even online editors like Photopea allow you to change colors, add outlines or shadows, adjust opacity, or apply filters. While PNGs are raster, you can make basic color changes or overlays. For significant shape alterations or infinite scalability, vector formats like SVG or AI would be better, or you can recreate the shape yourself.