the icons (bold, italics, align etc…) in editor mode can't be seen

On Editor Mode, the small icons on top of the text box where you can make the text bold, italics, align etc… can’t be seen. They are just silver squares with no “image” inside. The icons seem to work but I have to hover the mouse on top of it so I’ll know what button I am going to click.

I have a plugin called youtube embed and that’s the only icon that shows up.

I am using Google Chrome and I can’t make it to show up. However, in IE, everything works just fine.

Any idea how to solve this?

Thanks in advance!