Kubuntu allows themes to be changed simply. Go to the Kickoff Application Launcher (K Menu).

Left-click on Computer.

Open System Settings, thru Kickoff Application Launcher → Computer → System Settings.

In the Workspace Appearance and Behavior section, left-click on Workspace Appearance.

To install a new theme, left-click on the Get New Decorations... button on the bottom right.

From here, pick the source and run a search for the desired theme.

Once the theme is downloaded and installed, it can be picked from the list of installed themes.