That's why I did things like 
```
+  .icon-math:before {
+    content: "x";
+  }
+  .icon-math sup:before {
+    content: "2+1";
+  }
```
in menu.less and 
```
+  .icon-math *:before {
+    content: "";
+  }
```
in gh-book.less. The icons should look the same as before if gh-book.less isn't present. OERPUB icons shouldn't be required. I temporarily commented my new gh-book CSS specifically to view and test this. Your solution should work too (so feel free to recode it yourself :-p) ... both require gh-book to override stuff from the extra mark-up.