I just ran your sample, shared in #extgwt on IRC, and that is built with 3.0.0 (gxt.version is set to 3.0.0), and the stripped rows are working correctly, but the column lines are not. When I changed the GXT version to 3.0.1 (available in maven central), the column lines showed up.
As far as borders, here is the actual code sample from within org.opensheet.client.RequestFactoryTreeGrid:
tree = new TreeGrid<BaseProxy>(store, cm, cc1);
tree.setLoadMask(true); //TODO ?
tree.setBorders(true); //TODO ?
Switching this to false gets rid of the double border, and leaves only the single border that your FramedPanel adds. Changing from a FramedPanel to a ContentPanel (FramedPanel extends ContentPanel, and adds a border, aka a frame) then gives nearly the exact same view as the explorer sample.