White icons are invisible on buttons. http://maciej.kuzniczysko.pl/images/curseforge3.jpg Also good idea is create CSS icon sprite or even more - create own font (like Genericons in WordPress) for often used icons - it's easier to change their colors, less HTTP requests, but it won't work on very old browsers (however could be then displayed part of text).
Lightbox image browser is very old and unsupported. Check on landscape on mobile (by the way, sadly that website is not responsive). I suggest Magnific Popup. It's very good image browser. And when change you haven't change classes etc. just delegate from <body> to .lightbox
When creating issue I have only textbox to write tags. I created before "bug", so typed in ticket "improvement". But it won't saved. The same after edit ticket. I think that you would give reporters possibility to create own tags. If yes, good idea. If no, add multiple <select> dropdown to choose created tags.
Don't think about CurseForge as "it's only for computer purposes" - maybe someday you will expand to other sectors? Or there are people that sometimes prefer to check something on another device - for example answer some addon's feature request when lying on bed?
When you help them get better experience, they will like you more. Personally I am annoyed when have to use well known website on mobile or with veryyy old PC - usually it's just impossible to do something fast and with some kinds of websites, it just make me change for example news' website.