Hi Alberto,
Thanks for contacting Syncfusion support.
We were unable to reproduce the reported issue “my own css does not respect the change in the buttons and in the dialog line (blue color)”. We have generated custom CSS for Buttons and dialog title header color from the theme studio as per the documentation and refer the custom css in sample. Changes are applied to the buttons and dialog. While downloading custom css, you can get ej.widgets.core and ej.theme css files in minified and unminified format. So you need to refer both CSS files in layout.cshtml file.
For your reference, we have created a simple sample and the sample is available in:
Regards,
Rekha