How to apply HTML style?
How to apply HTML style? QOwnNotes - 22.6

The QTextBrowser of the preview only supports a limited set of html (see https://doc.qt.io/qt-5/richtext-html-subset.html). Best use the Custom note preview styling script from the script repository to inject styles.
In this case your problem is your code. It should be:
<style>
.divback {
background-color:brown;
}
</style>
<body>
<div class="divback">
авпвап
</div>
</body>
Note it's background-color
Hmmm. Actually it does recognise the background shorthand too.
I think your problem is due to you closing the CSS comment twice /* Цвет фона */*/. Remove one instance of */ and it works
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