Also...
<img src="{$ImagesDir}/custom/logo.jpg"> in the fashion_mosaic_grey folder should be
<img src="{$AltImagesDir}/fashion_mosaic_grey/custom/logo.jpg">.
Since you have already defined your skin any files in the common_files directory that are also in the (fashion_mosaic_grey) skin directory are no longer used.
Also be aware that the One Page Checkout and Fast Lane Checkout use different headers so you will need to change whichever one you use as well.
<img src="[$ImagesDir}/logo.jpg"> common_files folder.