Based from this guide on the ARToolkit website, the default marker border thickness is at 25%. I want to change it to 20% and also maybe by 15%. Is it possible to modify it? If possible, then how?
As you haven't specified the language I assume you are using ARToolKit in C. But the function to change the border width is also exposed to Android and iOS.
- mk_patt can now train markers with non-standard border widths. The command line switch "-border=n" (n between 0 and 0.5 (not inclusive)) specifies the desired border width as a proportion of the marker width. The portion of the marker which will be used as the pattern is now indicated by mk_patt by outlining in blue. Note that in order to use markers with non-standard border widths, the border width must be specified in the application by using the arSetBorderSize() function (see reference documentation for more information.)
(source: https://github.com/artoolkit/artoolkit5/blob/47d5796eef841e11f54e6d7040f389063c05b5a3/ChangeLog.txt)
Note: that the arSetBorderSize() is deprecated and you should use arSetPatRatio() instead
Here is the definition of the function you need to use: https://github.com/artoolkitx/artoolkit5/blob/47d5796eef841e11f54e6d7040f389063c05b5a3/include/AR/ar.h#L772
=== Edit 1: ===
As @Angelo Joseph Salvador now specified that he is looking for the solution of JSARToolKit it can be done using this function https://github.com/artoolkitx/jsartoolkit5/blob/master/js/artoolkit.api.js#L929
on an existing ARController object.