Anyway after the grumpy olde folks have had their rant lets try and do something unusual and answer the OP question :grin:
@Hund if you find Mint icons that you would like to use in place of the `geany-build` `geany-close-all` and `geany-save-all` you will need to replace the files that Mint installs of those names with `.png` and `.svg` extensions. For LMDE6 there are files of those names in `/usr/share/icons/Tango/*/actions` and `/usr/share/icons/hicolor/*/actions`.
Probably it would be better to check exactly what LM installs on LM21 since it might be different, I would use `synaptic`, select the `geany-common` package and `Properties` from the right click menu, then the `Installed Files` tab. There is probably some arcane `dpkg` command line you could sudo that does the same thing.
You have to replace each file with your new one of the same resolution, ie `...16x16/actions/geanu-build.png` needs to be replaced by your new one at 16x16 resolution, then all the other resolutions and a `.svg` in the `scalable` directory. I don't know if you could delete all the fixed resolution ones and just replace the `scalable` ones, but you probably have to regenerate the icon caches if you do. So all in all a lot of effort, not sure its worth it.