Preferences: Graph Options… to open the dialog to set default settings common to all future graphs.

Apply to button to apply the default settings to existing graphs, e.g. active graph, all graphs in folder, …
Click OK to apply to future new graphs.
Reset button to reset to preshipped Default, Classic and clear all to <auto>
System variable @GGO to control types of graph templates to apply settings
These numbers can be combined. E.g. set to 13 to apply the graph options when plotting graphs from built-in graph templates, app’s graph templates or downloaded extended graph templates.