A family of Microsoft word processing software products for creating web, email, and print documents.
There is no option built into Word to control display of the Styles Pane.
Remember, there is a keyboard shortcut to toggle display of the Styles Pane: Ctrl+Shift+Alt+S.
You could instead add the following macros to your Normal.dotm template.
Sub AutoOpen()
' Show Styles Pane in Versions 2016 and later (also works in 2010)
Application.TaskPanes(wdTaskPaneFormatting).Visible = True
End Sub
and
Sub AutoNew()
' Show Styles Pane in Versions 2016 and later (also works in 2010)
Application.TaskPanes(wdTaskPaneFormatting).Visible = True
End Sub
See https://www.gmayor.com/installing_macro.htm for information on what to do with macros.
You could use my free Add-In.
https://addbalance.com/word/download.htm#NavPane
It gives control over both the Styles Pane and the Navigation Pane. It uses the above dialog to let you set the default visibility and set the display width of those panes. It has its own keyboard shortcut to display the dialog.