HTC Touch Diamond – skin your calculator
While HTC has done wonders for the Windows Mobile UI, they (and Microsoft) have left the calculator app firmly in the Windows 3.1 camp.
Fortunately on Windows Mobile nothing so egregious need to be left unaddressed. You can give your calculator the Touch Diamond look simply by installing SpBCalc skinning app and copying the two skinning gif’s into your \windows directory.

Download the needed files from this XDA-Developers thread here.
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[...] WMPowerUser shows us how to modernize the look of the dated calculator application on the Touch Diamond. The same technique could be applied to other VGA and QVGA devices by downloading the file from XDA-Developers and following the instructions there. [...]