GUI: theme's default background images must be seamless

Some old themes like "default" or "coffee" uses old style background restricted to specific desktop sizes instead seamless border. It must be updated. Many software and ai tools can transform image to seamless (search like "create seamless texture").

Example:

Inspect the GUI theme assets for the "default" and "coffee" themes and compare how their background images behave across desktop sizes. Update the affected images so their edges tile seamlessly, then verify that the themes have no visible borders or gaps at different window dimensions.

Written by the indexing model from the issue text.