Hi ollie,
We can't set the style for the scrollbar in SfDataGrid based on the Windows 7 system style, as it is a custom scroll control. To change the appearance or theme of the scrollbar, you can set the theme for the SfDataGrid. Please find the code snippet below.
Code snippet:
this.sfDataGrid1.ThemeName = "HighContrastBlack";
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Output:

For more information, please refer to the list of themes mentioned in the user guide document linked below.
https://help.syncfusion.com/windowsforms/datagrid/themes
Regards,
Sreemon Premkumar M.