The answer depends on your window manager. The base WM, flwm, does not really support themes in a way you could package for other people, but there are a few simple configuration options that can be set. For example, the titlebar color can be defined with an environment variable (see the .info page in the repository for flwm.tcz). There are also few settings in .Xdefaults that may be experimented with, but you may find some settings unusable.
Other window managers like jwm, IceWM, FluxBox, OpenBox, have their own methods. You would need to study the WM documentation. Perhaps some current users of the alternative WMs can post their theme-related settings or files. But the first step is to pick a WM that you want to use and configure/customize.
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Mike L.