Exporting doesn't get custom CSS rules set in the software

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Describe the bug
The exporting of a document won't keep Custom CSS rules in the final document.

To Reproduce
Steps to reproduce the behavior:

1. Go to Extras
2. Click on Configuration
3. Scroll down to Custom CSS
4. Add new CSS Rules, for example a class changing the behavior of a div
5. Apply
6. Reset the software
7. On the top bar clic the < > (HTML) button
8. Add a class to the HTML tag of your text.
9. Go to File
10. Click on Export As ->
11. Select any of the options, (I have checked HTML, SVG, PDF, PNG)
12. Save your file to your PCExpected behavior
    The generated file should look as the file the program.

Screenshots

My file on the desktop My file on a PDF viewer.

draw.io version (In the Help->About menu of the draw.io editor):

• draw.io v29.7.9

Desktop (please complete the following information):

• OS: Windows
• OS Version 11, version 25H2

I tested the problem in incognito/private mode with all browser extensions switched off, write "yes" below:

• N/A as I'm using the desktop version

Additional context
Add this code so the MWE can work.

.cuadro-amarillo {
    margin-right: 5px;
    margin-left: 5px;
    font-family: Arial, Helvetica, sans-serif;
}

.cuadro-amarillo .titulo {
    padding: 5px;
    text-align: center;
    font-weight: bold;
    border: 1px solid #deb80e;
    border-bottom: 0px;
    border-top-left-radius: 6px;
    border-top-right-radius: 6px;
}

.cuadro-amarillo .cuadro-texto {
    padding: 5px;
    border: 1px solid #deb80e;
    line-height: 0.9;
    border-bottom-left-radius: 6px;
    border-bottom-right-radius: 6px;
}

.cuadro-texto ul {
    margin-top: 10px;
    margin-bottom: 1px;
    padding-left: 20px;
}

.cuadro-texto p {
    margin-top: 4px;
    margin-bottom: 1px;
}

MWE.drawio

Start by reproducing the export behavior with MWE.drawio, using the Custom CSS configuration and the File > Export As path described in the issue. Trace how custom CSS is carried into HTML, SVG, PDF, and PNG exports. Done means exported files retain the configured rules and match the editor's appearance.