Add distinct CSS class to `<img>` tags

[astrojuanlu]

I see that a custom class is being added when the HTML builder is in use:

sphinxcontrib-mermaid/sphinxcontrib/mermaid.py

Lines 231 to 239 in 050452e

	if "align" in node:
	tag_template = """<div align="{align}" class="mermaid align-{align}">
	{code}
	</div>
	"""
	else:
	tag_template = """<div class="mermaid">
	{code}
	</div>"""

However, the PNG builder does not add any. The code is ready for it:

sphinxcontrib-mermaid/sphinxcontrib/mermaid.py

Line 272 in 050452e

imgcss = imgcls and f'class="{imgcls}"' or ""

But it's never being used:

sphinxcontrib-mermaid/sphinxcontrib/mermaid.py

Lines 291 to 292 in 050452e

	def html_visit_mermaid(self, node):
	render_mm_html(self, node, node["code"], node["options"])