www.wvit.net

For editing the default Style Sheet or selecting it, you can for example >>Edit>>Style Sheets, push the "New" button, change the Name to Default, use Fancy or any others as the template and select "OK" it will overwrite the "Default" and now you can have your logo or whatever pop up by default.

For changing the default words I use the command line:

sudo sed -i 's/"Thank you for your patronage"/"What do you want it to say"/g' /usr/local/share/gnucash/guile-modules/gnucash/report/invoice.scm

If you're not using linux, you can accomplish the same by editing the invoice.scm file, wherever it is on your system.

Hope this will help you