Does it not work at all or only in part? I ask as you may need to change more than the body text to cover all elements.
Here is a part of my userstyle.css file that covers some printing / pdf export settings. I use these to control the font size and make the output as monochrome as possible. You can use it as a starter to get the look you want.
Please note - After editing userstyle.css you have to fully quit Joplin (File > Quit) and restart it again before the changes will have any effect.
@media print {
body {
font-size: 12px;
color: #000000;
}
table th, table td {
font-size: 12px;
color: #000000;
border: 1px solid #000000;
/* black table borders for printing */
background: #ffffff;
/* white table background for printing */
}
a {
color: #000000;
text-decoration: none;
/* remove link underlines for printing */
}
.resource-icon {
background-color: #000000;
/* make resource icons black for printing */
}
blockquote {
border-left: 4px solid #000000;
opacity: 1;
/* make quote marker border black for printing */
}
code {
font-size: 12px;
color: #000000;
font-weight: bold;
/* bold code black text for printing */
}
.inline-code {
font-size: 12px;
color: #000000;
border: 0px none;
background: none;
font-weight: bold;
/* bold code black text with no background for printing */
}
pre.hljs {
border: 0px none;
background: #F0F0F0;
/* remove fenced code border and modify background colour for printing */
}
hr {
border-bottom: 1px solid #000000;
/* change horizontal rules to black for printing */
}
h1 {
border: none;
border-bottom: 1px solid #000000;
/* change horizontal lines under h1 tags to black for printing */
}
}