Hi @alex4444,
The scrollbars with *-base.css theme are styled only when the component is placed inside a RadzenLayout (a container with .rz-layout css class).
We managed to reproduce the issue with scrollbars not being styled in RadzenDropDown. It happens because the dropdown's popup is not rendered inside a RadzenLayout container. Perhaps this is the issue @Anjali_Menon also observed not placing the RadzenDataGrid inside RadzenLayout.
Since the .rz-layout container adds additional layout sizing and positioning styles that you might not need, we are considering the introduction of an optional .rz-scrollbars css class in base themes that will allow you to style all scrollbars by adding it to the <body> of you app. The class will be available with the next Radzen.Blazor version.
Alternatively you can use a non-base theme e.g. default.css, but chances are it will also override other styles in you app such as font-size, line-height etc.