I added in a Glow filter and a new Frost UI theme that blurs the background behind UI windows. I had to make some minor modifications to the jME and Nifty sources so that I could use my own shaders on NiftyGUI Elements.
The new Frost theme and the Glow effect can be turned on/off through the options menu. The Glow effect has three settings, off, low and high. The low setting renders two of the game world cameras at half resolution with a custom Technique and applies a two pass blur shader. The high setting renders the two cameras at full resolution and applies a four pass blur filter. I did not use the built in FilterPostProcessor, this way I didn't have to clear the original scene buffer and re-write to it, instead I just switch the renderer back to the main buffer and overlay the glow effect using a BlendMode Additive shader.
The Frost theme can be set to off, low or high as-well. On the low setting the UI window elements use a single pass blur shader, UI elements are not blurred or displayed behind other UI elements on the low setting. Under the high setting each UI window or element with the shader applied is rendered with 5 passes, 4 blur passes plus an additional pass to overlay the window texture. UI elements are blurred and rendered behind one another on this setting.
Both the high and low Frost settings also uses a normal map for the UI shaders which wraps the background image around the edges of the windows giving the appearance of refraction. The Frost theme is not a full screen filter, each element that needs to be blurred is rendered using a shader so only the area of the screen that needs to be blurred is processed.
P.S. I'm really pleased with how the new UI theme turned out, unfortunately my laptop is too slow to run it at playable speeds , but my machine isn't exactly a gaming rig. With both the Glow and Frost set to high I get about 7 fps with larger windows open, but I imagine more modern machines with decent graphics cards in them should be able to run it just fine.