I don't know if you saw the comments from ninetrydegrees and timeasmymeasure, but you don't have to have a paid account to do the necessary copying and pasting to use this style - that was my mistake, sorry! (Of course, if you want to get a paid account anyway then no one is going to stop you!)

Here's what you do:

1) Go to: http://www.dreamwidth.org/customize/advanced/layers
Under "Create top-level layer" select Type: "Layout" and Core Version: "2". Hit "Create".

2) Under "Your Layers" a box should have appeared with LayerID, Type, Name and Actions. Press "edit" next to the new layer you just created (I guess it will be the only one).
Copy and paste the contents of http://www.dreamwidth.org/customize/advanced/layersource?id=129256&fmt=html into the main window (deleting what's already there).
Press the "Save and Compile" button at the top of the screen.
Now click "Back to Layers List".

3) Under "Create a style-specific layer" select Type: "Theme" and Style: "Boxed In (#number)". The number will match the LayerID number in the box above.
This should create another entry in the box.

4) Click "edit" next to the new layer (type: theme, name: (none)).
Copy and paste the contents of http://www.dreamwidth.org/customize/advanced/layersource?id=235266&fmt=html into the main window (deleting what's already there).
Press "Save and Compile" and then "Back to Layers List".

5) Go to: http://www.dreamwidth.org/customize/
Select "Your Custom Layers" from the panel on the left.
Pick "Spring Fling".

6) ENJOY!

I hope that makes sense. If anything is not clear or does not work then please let me know and I will clarify.