Complete beginner here using Calligra on a Ubuntu flavour. I messed-up the layout of a spreadsheet and could not undo it so I re-installed the whole suite. This made no difference and so I deleted the .kde folder. Now, when any of the programs start, they have a very old-fashioned look to them (almost like a 90's Sun!) and uninstalling/re-installing does not help. Sorry, no idea what I have done wrong. All advice much appreciated. It seems to be a lovely suite of programs if you just want a light-setup.

Hi bat. I am assuming you had no backup to restore the .kde folder. Doing so is like cleaning up Windows registry - you can lost all settings. But here at least all apps and the workspace are just back to the default setting.
I don't know why your default style is like that now but you can use KDE system settings (https://userbase.kde.org/System_Settings) to restore style and possibly icon theme.

For the future, to reset configuration of an app, just remove (or backup and remove) one file: ~/.kde/share/config/{APPNAME}rc
For Calligra Sheets it's ~/.kde/share/config/sheetsrc.

PS: For Calligra 3 it will be ~/.config5/{APPNAME}rc.

Thank you, man, I should have said that it is only the calligra style/format that is changed. The rest of the programs on my system have retained their 'Lubuntu' theme. There must be some settings file that I don't know about yet so any clues would be much appreciated.

OK. I expected you're not running the KDE plasma desktop, which is fine of course.
I don't think it's for Calligra only. You may want to check other Qt/KDE apps such as dolphin or kwrite to see how they look.

Your other apps that look of for you may be not Qt apps, that's why they are displayed as expected -they are native to your Desktop.

Calligra apps are Qt apps that inherit look and feel from Qt's settings.
You may try to run a 'qtconfig' app and set the default style. The corresponding config file that you can edit or even remove is ~/.config/Trolltech.conf.

I recommend asking the 'Lubuntu' (LXDE) developers to look at the issue with integration, if you find some more issues, it's largely their job. They're moving to Qt anyway (with LXQt), so it's even more important for them I guess to integrate. Updated KDE apps (like soon Calligra 3) are becoming "just" Qt apps these months, with better integration with Qt-based desktops (Plasma, LXQt).

KDE desktop software aligns well to many 'foreign' apps - even offers them a free styling plugins (for GTK+), and icons/styles for LibreOffice. It would be good to see similar openness on the other side. Requests from users can help in this process a lot.

Thank you again, man, your last reply had the critical clue - I had foolishly deleted the Trolltech.conf file and when it was re-created by Calligra, it lacked the 'Lubuntu' settings. Anyhow, after a bit of Googling, I added the following three lines to the top of the file:
[Qt]
style=GTK+

and now Calligra looks like it should again. Many thanks for your help with this.

Good. The GTK+ style is exactly the one responsible for the integration of Qt apps with the GTK+ look&feel - something I was talking about

Feel free to report your thoughts from your usage of Calligra apps
Cheers.

J