The 'about KDE' in Help on various KDE applications doesn't show the version number but as far as I can tell from the package manager, the version is 5.8.6. The operating system is PCLinuxOS.

I have tried both the Fusion and Oxygen widget styles and they are NOT applied. That is most notable with the boxes marking a selection. Both those styles use a tick or check mark for a selection but the Breeze style (which instead colours boxes) continues to apply. The option to change widget styles is in System Settings > Appearance > Application Style > Widget Style.

I have tried several colour schemes to no avail. The system continues to use the Breeze colour scheme. The setting is in System Settings > Appearance > Colours. One of the styles I have tried is Oxygen.

The desktop theme I am using is Oxygen.

I am posting here prior to lodging a bug report.

Which applications? All applications? KDE/Qt applications? GTK applications (e.g. Firefox/Chrome/ium, Synaptic, Configure My Computer)? Applications running as Root (Synaptic, Configure My Computer)?

GTK applications follow a separate config and currently the Breeze GTK style doesn't respect the color scheme defined (there have been a couple of branches for doing that, but last I knew none of them has been merged in to the default). Instead you should be able to configure the GTK style to select between Breeze, Breeze-Dark, and a few other GTK-specific styles (I don't remember off-hand where that config is in KDE 5, or what you might need to install to enable it in PCLOS. I'll check up on this when I get home this evening).

Root applications can be coerced to run using the currently-defined color scheme. I don't know the steps off-hand, but there have been discussions around this on this and/or the PCLOS forum. I believe you should be able to run systemsettings as root and then configure the appearance for root applications that way.

Thanks for replying. Soon after posting I was unable to load the forum in my browser.

I hadn't realised that the setting for Widget style applied only to KDE applications.

Since posting I have installed gtk-oxygen-molecule which is available as a GTK2 theme. Setting it as a theme had the effects of changing colours and widgets to what I hoped to achieve.

To get an oxygen theme for GTK 3, I installed oxygen-gtk and oxygen-gtk3 which results in gtk-oxygen-molecule being removed. Unfortunately, after I applied the new themes two unwelcome results followed. First, the context menu in Firefox became compressed; secondly, there was a wholesale replacement of my carefully chosen application launcher icons (I have 27 on the panel); I reinstalled using my own remaster. I have set up another system for further experimentation with themes.

The issues are solved.

For the record, I changed to oxygen-gtk and oxygen-gtk3 on the test system. Icons were unaffected but ALL menus in Firefox became compressed. In addition, the buttons on the Firefox profile manager appeared as text, in other words, without any button outlines.

The effects on Firefox occur even if only one of those themes is used.