You can just use normal HTML tags, here is an example of my “Device” template, which includes a red or green text (down vs. recovering), some device info and a traffic graph. It is pretty simple and I plan to improve on it in the future, just didn’t get around to it yet.
<div style="font-family:Helvetica;">
<h2>{if %state == 1}<span style="color:red;">%severity{else}<span style="color:green;">recovering{/if}</span></h2>
<b>Device:</b> %hostname<br>
<b>Location:</b> %location<br>
<b>Uptime:</b> %uptime_long<br>
<b>Duration:</b> %elapsed<br>
<b>State:</b> %state<br>
%title<br>
<br>
%description<br>
%features<br>
%purpose<br>
%notes<br>
{if %faults}
{foreach %faults}%value.sysDescr<br><br>
<img src="https://monitoring.librenms.example/graph.php?width=459&height=213&lazy_w=552&from=end-24h&device=%value.device_id&type=device_bits&legend=yes"><br><br>
{/foreach}
{/if}
If I remember correctly, you have to activate unauthenticated access to the graphs for this to work in your email client.