Hi Tomas,
Once you have set a style.ReadOnly = true;, you must then set grid.IgnoreReadOnly to be able to change it.
Please try this sample.It seems to be working fine.
RowStyleOverrideColStyle.zip
Please refer this sample for more details:
C:\program files\syncfusion\essential studio\3.3.0.0\Windows\Grid.Windows\samples\Quick Start\GridStyleInfoAtWork
Let us know if this helps.
Best Regards,
Jeba.