Workaround for XHTML not supporting <picture>

testing/028_formula.c

@@ -1,17 +1,18 @@
 // objective: test the \f$, \f[, \f], \f{, and \f} commands
 // check: indexpage.xml
+// config: HTML_DARKMODE = LIGHT
 
 /** @mainpage
 Here are some formulas:
--# The distance between \f$(x_1,y_1)\f$ and \f$(x_2,y_2)\f$ is 
+-# The distance between \f$(x_1,y_1)\f$ and \f$(x_2,y_2)\f$ is
   \f$\sqrt{(x_2-x_1)^2+(y_2-y_1)^2}\f$.
 -# Unnumbered formula:
-   \f[ |I_2|=\left| \int_{0}^T \psi(t) \left\{ u(a,t)- \int_{\gamma(t)}^a 
+   \f[ |I_2|=\left| \int_{0}^T \psi(t) \left\{ u(a,t)- \int_{\gamma(t)}^a
        \frac{d\theta}{k(\theta,t)} \int_{a}^\theta c(\xi)u_t(\xi,t)\,d\xi \right\} dt \right| \f]
 -# Formula in different environment
-  \f{eqnarray*}{ g &=& \frac{Gm_2}{r^2} \\ 
+  \f{eqnarray*}{ g &=& \frac{Gm_2}{r^2} \\
                    &=& \frac{(6.673 \times 10^{-11}\,\mbox{m}^3\,\mbox{kg}^{-1}\,
-                       \mbox{s}^{-2})(5.9736 \times 10^{24}\,\mbox{kg})}{(6371.01\,\mbox{km})^2} \\ 
+                       \mbox{s}^{-2})(5.9736 \times 10^{24}\,\mbox{kg})}{(6371.01\,\mbox{km})^2} \\
                        &=& 9.82066032\,\mbox{m/s}^2
  \f}
 */