Override hard-coded color in \opt for dark mode

[timsong-cpp]

\opt generates something like (linebreaks added for readability)

<span class="mjx-mstyle" style="font-size: 113.1%; color: black;">
<span class="mjx-mrow" style="font-size: 88.4%;">
<span class="mjx-mi">
<span class="mjx-char MJXc-TeX-main-I" style="padding-top: 0.225em; padding-bottom: 0.298em;">
o
</span>
</span>
</span>
</span>
<span class="mjx-mi">
<span class="mjx-char MJXc-TeX-main-I" style="padding-top: 0.225em; padding-bottom: 0.446em;">
p
</span>
</span>
<span class="mjx-mi">
<span class="mjx-char MJXc-TeX-main-I" style="padding-top: 0.372em; padding-bottom: 0.298em;">
t
</span>
</span>

so the o remains black even in dark mode.