I've put this in the userstyle.css for random sections to emphasize certain text. This works fine for me in the dark theme, but I think it works well in a light theme as well.

Dark:

Light:

:root {
  --white: #e9edf0;
  --yellow-dark: #E7AE1D;
  --blue: #02A4F1;
  --purple-dark: #F459F4;
  --red-dark: #FF1744;
}

note::before,
info::before,
question::before,
warning::before
{
  font-weight: 700;
  text-shadow: 1px 1px 3px #000000;
  color: var(--white);
  margin-right: 0.2rem;
  position: absolute;
  width: 2rem;
  left: 0;
  top: 0;
  height: 100%;
  display: grid;
  place-items: center;
}

note,
info,
question,
warning
{
  display:block;
  padding: .5rem 1rem .5rem 2.5rem;
  position: relative;
  margin: 15px 0;
  border-width: 1px;
  border-style: solid;
  border-radius: 10px;
	overflow: hidden;
}

note {
	border-color: var(--blue);
}

note::before {
	content: "\266A";
	background: var(--blue);
}

info {
	border-color: var(--yellow-dark);
}

info::before {
	content: "\2139";
	background: var(--yellow-dark);
}

question {
	border-color: var(--purple-dark);
}

question::before {
	content: "?";
	background: var(--purple-dark);
}

warning {
	border-color: var(--red-dark);
	font-weight: bold;
}

warning::before {
	content: "\26A0";
	background: var(--red-dark);
}
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