There are 3 basic scenarios when you want to modify default settings:

you are system administrator and get a dozen new users every day. You want that the new users have certain settings that are not default for a KDE Plasma Desktop
you are system administrator and want to change the desktop or system settings of all your users. Logging in as a user and do the changes in System Settings is not an option.
you have several KDE desktop machines; maybe a computer at work behind a proxy, a machine at home and some virtual machines for testing. You re-setup computers frequently and want your settings to be rolled out again afterwards

In any case you will have to make the setting files in ~/.kde or ~/.kde4 or /usr/local/share or /usr/share change.

/etc/skel

When a new user is created, the content from /etc/skel is copied to his home directory. If you want to change the settings for all future users, just change the files in /etc/skel.

examples

To change the default browser, use the program systemsettings. However, there are situations where you cannot call systemsettings. In this case you can use the following script. It changes the default browser to firefox:

#!/bin/bash
# This file sets up a computer as tstaerk likes it. Call it after you have performed the OS installation.

# 1. set default browser to firefox
# 1.1. for SUSE Linux, where we have to look in .kde4 instead of .kde
# The key for default browser is BrowserApplication.
cd
cd .kde4/share/config
# first delete all the old entries for BrowserApplication
sed -i 's/BrowserApplication\[\$e\]=.*//g' kdeglobals
# add BrowserApplication to the General section if a General section exists
sed -i 's/\[General\]/\[General\]\nBrowserApplication\[\$e\]=!firefox/g' kdeglobals
# add a General section and the key BrowerserApplication if no General section exists
grep "\[General\]" kdeglobals || echo -e "\n[General]\nBrowserApplication[\$e]=!firefox" >> kdeglobals