It turns out that it could be through the use of a themes website. This is according to a Redditor who posted an alleged email exchange they had with a Samsung rep. When asked when the tool would be released, the rep replied by confirming that the tool has not gone live yet, at least for the Galaxy S6 and that it is only available for the Galaxy Note 4 and the A-series in China.
However the rep also confirmed that the page will be “renewed” in April where the authoring tool for the Samsung Galaxy S6 will be added, which more or less confirms that we can look forward around that time, assuming that the rep was on point with their answer. Theming tools are not new and if you want to theme your phone, you can always turn to third-party options such as MyColorScreen’s theming app while you wait for the official Samsung themer to be launched for your Galaxy S6.