Without knowing which theme you use or which plugin we cannot really tell you much. Even if we knew what theme it is it would be hard as we may not know how the theme is coded.
What I can tell you is that the alt text on links is definitely not in any of the CSS files, as it’s not what CSS files are for. You have to look in template or theme function files. If you have access to your server via FTP you could download the theme and run find command on the theme directory (some text editors offer an option to find a string in all files from directory). You will then know which file and line the text is located at.