Using Linux Mint 17. Installed KDE as an alternative to the existing Cinnamon setup using:

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apt-get install kde-plasma-desktop

Logged into KDE, realized the problem (although everything else seems to be working just fine - all window effects, etc.), made some minor cosmetic changes, failed to set it right, went back to Cinnamon. Found out that Mint has a metapackage for installing KDE, so used:

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apt-get install mint-meta-kde

Problem persisted, so logged out, went back to Cinnamon, and did this for a hard reset:

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mv ~/.kde ~/.kde.old

Did not work, so went back to Cinnamon, and tried this:

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apt-get install --reinstall kde-plasma-desktop

UPDATE: Covered for oversight - tried this too:

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apt-get install --reinstall mint-meta-kde

No change in situation.

NOTE: By "minor cosmetic changes", I mean changing the theme and generally tweaking every relevant setting I could find in Workspace Appearance and Behavior in System Settings, but to no avail. For example, theme changes are reflected in the title bar and the system panel, but not inside windows.

Did you install gtk2 and 3 engines?

Used this:

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apt-get install gtk2-engines-*
apt-get install gtk3-engines-*

Installed a bunch of stuff, but no change on login to KDE Plasma.

Logged out, tried this:

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mv ./.kde ./.kde.old

... then this:

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apt-get install --reinstall mint-meta-kde

... but still no change.

Led me to wonder if I must reinstall Qt, but did not find a PPA for the the latest non-dev version. Any suggestions?
Also, how do I query the installed Qt version?

Please try removing ~/.config/Trolltech.conf

Resolved. Many thanks, @bcooksley, @toad.