I have /var/tmp/kdecache-username taking up a whooping 5gb space. Most of it is about plasma theme files (I had downloaded and tried several from Get Hot New Stuff).

They persist even on reboot.

Is it safe to delete these files? In general, is it safe to delete files in /var/tmp/kdecache-username?

As far as I'm aware yes, although maybe it's a good idea to not do it while KDE is running. There's no valuable data stored there, though.

I have actually deleted the files in /var/tmp/kdecache-usarname and it has not done any apparent damage.

However, is not supposed to be 'auto-clean'ed?

Yes, kind of. Did you look what data exactly took so much space?

OK. I had deleted files from earlier lot.

But the new ones keep cropping up as and when I change the desktop theme. So, I have a few of them already.

The filenames are: plasma_theme_themename.kcache. Each file is about 80 MB. I tried opening a sample file in vim. But could not.

Hope this helps.

my understanding is that the files are caches, they are used to save time in rendering the plasma themes (which are svg), every time you install a new theme a new cache for that theme is created