Bug Reports: Re: Slider glyphicon chevron displays as square
The problem is that becuase the modified theme is stored in your assets folder, the path to the glyphicons file no longer works.
The css is looking for ../fonts/glyphicons-halflings-regular... but the files aren't in this location relative to your CSS.
If you edit your custom css and look for references to glyphicons files, and change the path to be the absolute path /resources/themes/bootstrap3/fonts/glyphicons-halflings-regular.<ending> then it will find the font files and the icons should display correctly.
I changed the relative path to point to my custom css as you advised and replaced the existing paths in the @font-face with the new path. All working fine now.
Thanks for your help.