warning: Invalid argument supplied for foreach() in /.../themes/fusion/fusion_core/theme-settings.php on line 129.

i guess, you are using the domain access module as i do, or have another module with theme settings page, where the theme name is not the last one in the argument list, as fusion expects.

the problem is, that phptemplate_settings in fusion core define $theme_name variable as the last item from the argument list - arg(count(arg()) -1 - what gives the "theme-settings" value, what is not a valid fusion subtheme name.

general theme settings url:
admin/build/themes/settings/[theme name]

domain access module with domain theme:
admin/build/domain/theme/[theme name]/[domain id]/theme-settings

i dont know, if its really a fusion or a domain access issue - if domain access defines the path in a non-standard way -, my current solution is a small hack, what checks if argument list contains domain or not, and select the correct argument according to the result. (what is really a hack as who knows, what other module will have theme setting page in the future..)

is there a better solution?

See this issue: #691026: How do I remove the theme settings from a theme that used fusion before?.
That suggestion didn't work for me however, I'm using LightFantastic and I'm getting this error. I didn't use other Fusion themes on this site before.

but as you still use a fusion subtheme and i guess, the problem is in the fusion_core..

Changing to Fusion project since this issue is related to fusion_core.

Closing as won't fix since the D6 version is not getting any new development.