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System Settings -> Colors (Application Color Scheme).
We can set specific the color (one color). It would be nice to set a gradient. like this #1e5799 0%,#7db9e8 50%,#1e5799 100% (three colors). It is better to make an infinite number of colors.
Add Gradient Support

There are alternative theme engines which allow setting up such custom gradients, including QtCurve (https://github.com/KDE/qtcurve), StyleProject (https://sourceforge.net/projects/styleproject/) and Kvantum? (https://store.kde.org/p/1005410/). For QtCurve and StyleProject at least, the gradients are defined as a percent lighter/darker which is applied to the color of the element. With Kvantum, the theme supplies its own sets of SVG assets which you may customize to your heart's content (and they generally ignore whatever color scheme you might have defined in system settings, instead providing their own). You can select which one you want to use in System Settings in the (Widget) Style page.

Of these I'd recommend trying QtCurve as that should be either installed by default or easily installable from your distro's repos and is the easiest to configure using its provided GUI.

(this is of course ignoring desktop themes which like Kvantum themes are comprised of SVG assets which you may customize to your heart's content, and most but not all of which ignore the color scheme defined in system settings)