Here is the snippet I just added to the User Guide.
Code:
<li><p><span class="listheading">SIGIL_USE_COLORSCHEME_CHANGED</span>
This environment Variable is <b>Linux Only</b>. Setting this tells Sigil that your
Linux desktop works with Qt's built-in QStyleHints::colorSchemeChanged signal
for detecting when your desktop theme changes from light to dark and visa-versa.
</p></li>