Fixed replacing URLs in CSS for something like data:image/png

When CSS file contains something like this
```css
background-image: url(data:image/png;base64,iVBOoAAAAN…wcUVORK5CYII=);
```
there were incorrect replacements with
```css
background-image: url(ff0a9447/css/data:image/png;base64,iVBOoAAAAN…wcUVORK5CYII=);
```