The forward/backward shortcuts work because they are related to the browser's function, but if you're looking for a 'previous' and 'back' shortcuts related to pages within a thread that might not be as likely. 'Previous' and 'back' links don't exist on the forum for navigating within a thread. There are 'previous/next' links that take you to the next topic in a subforum, but there aren't ones related to a particular thread. You navigate a thread by going through to the page number. There would have to be a shortcut for each page number, which wouldn't make sense for a thread that has dozens or hundreds of pages. And that's saying that you would be able to program a shortcut to find a certain link. That might or might not be possible, but it doesn't seem like it would make sense in terms of the layout of the forum.
If you only wish to use your keyboard though, and if you're using chrome you can use the tab button to move forward and shift+tab to move backward, and then press enter once you land on the desired link. There is also a google chrome extension called "Type-ahead-find" which allows you to quickly search for links by using the single quote ' and then you type what you are looking for. It uses the tab order that was just mentioned so when you land on a link you are able to just press enter and go to that page. If you were on page one of a thread on the forum you would type '+2+enter to go to page 2. It's not exactly a shortcut, but it would be pretty quick.