Here is a demostration:
(Those who can read Chinese may find it more `readable' than the original ones...)
Before/After:

It's quite simple.
Know that two images are used for rune icon (as css background):
Normal-sized ( .rune ):

Mini ( .minirune ):

Changing them will be enough to make things happen.
(To to that your browser needs the `user styling functionality'.
If you're using Mozilla Firefox, you can use the extension
Stylish or
Greasemonkey;
Otherwise
this link might be helpful. )
Now it's the details of image editing:
The size of a normal rune icon is 40x40, 4px padding, 1px inner border, 31x31 content image;
The mini rune icons are just 20x20 squares, no border.
The new icon images I made are 32x32 squares, like these:


(Actually it shall be 31x31, I forgot the 1px border at that time. But no matter, I'll change it later.)
Then I use
ImageMagick to composite the two mask images I need:
montage icons/*.png -geometry '30x30+5+5' -tile 5x4 -background transparent rune_mask.png
montage icons/*.png -geometry '18x18+1+1' -tile 5x4 -background transparent minirune_mask.png
(The params of the `montage' command above is easy to understand:
-geometry '30x30+5+5' means that all images will be resized to 30x30 plus 5px borders,
-tile 5x4 means that we need 5 per row, total 4 rows of these images combined.
Refer to the ImageMagick manual for more details.
If you're not familiar with the command line, you can always use your favorite image editor to do the job.
Just remember to make the sizes right.)
Then we get:


Let's make some `modifications' to the original images:


And put them togather:


Now We could use
data URI scheme(as in
here) or just upload the images to some where and use them, like:
.rune { background-image: url(http://bubble-hearth.appspot.com/image/runes/release/rune_comp.png) !important; }
.minirune { background-image: url(http://bubble-hearth.appspot.com/image/runes/release/minirune_comp.png) !important; }
.minirune_null { background-image: none !important; } /* don't forget these two lines, or the default icon will be shown in where no icon shall be shown */
.rune_null { background-image: none !important; }
And load it into our browser (using Stylish, Greasemonkey, etc.).
Thanks for reading. Have fun.
----
Btw, several days ago, I made a `fancy version of estiah's charactor banner', maybe some of you would like to try it.
It's like this:

Just avatar + default banner + hp, sp stats.
To use it, just make sure you have default banner image enabled at
here, and than this will be the image:
http://bubble-hearth.appspot.com/estiah/<Your Charactor ID>
(The data of the image will be cached for half a day, so in theory it'll only be 1/2 day outdated.)
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Sorry for my bad English. Well, you know, I'm new to Earth...