Select cells

It may depend on what you'd like to do after selecting the cells but you could use Find/Replace "choose format from cell".

The functionality is described near the end of this article:

Find or replace text and numbers on a worksheet - Microsoft Support

You could use a small VBA routine:

Sub FindaStyle()
    Dim oSh As Worksheet
    Dim oCell As Range
    For Each oSh In ThisWorkbook.Worksheets
        For Each oCell In oSh.UsedRange.Cells
            If oCell.Style Like "*demo*" Then
                Application.GoTo oCell
                Stop
            End If
        Next
    Next
End Sub

Taken from my website article on styles: https://jkp-ads.com/articles/styles.aspx