There may be something going on with the native themes service being prevented from regaining control.
Have you tried the following:
1 - Unload WindowBlinds
2 - Display Properties > Themes tab > select Windows Classic > click Apply > click OK to close > reboot computer
3 - After computer reboots, Display Properties > Themes tab > select Windows XP > click Apply > click OK to close > reboot computer.
Note - if you are using Icon Packager, you may want to do a quick "Tools > Restore all Icons > "Yes" to confirm > "OK" to apply changes, before performing steps above.
I have noticed that WB5 will intermittently hold onto something that prevents the Luna theme from being applied, and the steps above seem to clear it.
Note #2 - this assumes that you have not disabled the "themes" service in "Control Panel > Admintrative Tools > Services" for some reason.