Border outline with multilevel headings

Hi everyone,

I am building out dash with a table display and several graphs. My partners want a table to be in a specific format for communication to the leadership. Looking through this forum I have found a lot of solutions to my questions, so first off want to thank everyone for your input so far!
The adjustment I am trying to make right now is to get bold/dual border around sections under first level header. Unfortunately, I cannot share an exact view of my own example due to proprietary data. Closest example would be something I took from Dash website (code below). It is located under ‘Multi-headers’ section here: Styling | Dash for Python Documentation | Plotly. Here I would like to know how I can add a heavy border (1.5-2px) but only around the group under ‘City’, not a grid for the section, just outline border.

{“name”: ["", “Year”], “id”: “year”},
{“name”: [“City”, “Montreal”], “id”: “montreal”},
{“name”: [“City”, “Toronto”], “id”: “toronto”},
{“name”: [“City”, “Ottawa”], “id”: “ottawa”},
{“name”: [“City”, “Vancouver”], “id”: “vancouver”},
{“name”: [“Climate”, “Temperature”], “id”: “temp”},
{“name”: [“Climate”, “Humidity”], “id”: “humidity”},
“year”: i,
“montreal”: i * 10,
“toronto”: i * 100,
“ottawa”: i * -1,
“vancouver”: i * -10,
“temp”: i * -100,
“humidity”: i * 5,
for i in range(10)

Thank you in advance!