Hi Geethaka Fernando,
Thanks for the question. At this time, Arcade expressions have limited support in Operation Views (e.g., Arcade expression attributes display in the content of pop-ups and Details elements). It is in our backlog as a consideration to enhance the support of Arcade expressions to more visual elements.
Regarding the code from that video click, the following dashboard uses the tag technique you are referencing. In the menu in the top right you can create a copy of the dashboard. Configure the List element, go to the List tab, edit the List Item Text, and click <> Source to see the code and preview.
http://www.arcgis.com/apps/opsdashboard/index.html#/37d0e7637f98479e83f5cdcf51038c3e
I'll paste it here too.
<div>
<p><span style="background-color:{advisory_color}; border-radius:3px; color:#242424; float:right; font-size:11px; padding:1px 4px 1px 4px">{advisory_text}</span></p>
<div style="float:left">
<p><span style="font-size:14px">{WATERBODYNAME}</span></p>
<p><span style="color:#b8553b">{ADVISORYDESC}</span></p>
</div>
</div>
At this time, Arcade expressions are not supported in the List element. You would have to reference an actual field with the hex color code.
Hope this helps!