how can i make it so that when i change a theme, the wallpaper doesn't change. all the wallpapers are either poor quality, or the wrong size for my needs, or simply not something i want. but i find that if i apply a wallpaper, then apply a theme, it changes and i don't want it to

Personally i right click on the startbar and choose "Nextstart Preferences". Then i click on the "Theme Management" tab. Then, i click on "Theme Options" and then check "Lock Wallpaper". Then click ok, apply and ok to exit all the open preferences windows.

i tried that, i found that in the help documentation, but it doesn't work for me, it still applies the theme specific wallpaper

Please specify, in step-by-step detail, what you are doing to 'lock the wallpaper' and then to apply a theme which still changes your wallpaper.