You basically need to upload the folder (and contents) to your styles directory (you will see subsilver and prosilver in there already) and then you just need to go to the style section of the acp and activate it.
It means that you probably didn't upload it right. The path needs to look like styles/stylename/templates, styles/stylename/theme and so on. If it is like styles/stylename/somethingelse/templates it won't be picked up. Basically when viewing the styles directory, you should see prosilver, subsilver and you new style listen. When you click the new style, you should then see the template, theme and imageset directories.
Delete the /acidtechgreen/ directory in the /styles/ directory and re-upload the /acidtechgreen/ directory that contains the theme, template, and imageset folders. In other words, the path should be /public_html/forums/styles/acidtechgreen. Within that folder, there should be /public_html/forums/styles/acidtechgreen/theme, /public_html/forums/styles/acidtechgreen/template, etc.
In other words, it shouldn't be /acidtechgreen/acidtechgreen.
If you open the folder on your desktop, it should contain an acidtechgreen folder. Upload that one.