It is currently possible to create a custom style where most of the style is inherited and minor changes are made to it by board owners. This makes it easy to insulate future changes to the inherited style from minor overrides in the custom style.
The same is not true with template events. One cannot create a /template/event folder under the custom style and use any template events. You have to write an extension to do this, something likely beyond the technical capabilities of board owners. The alternative is to copy templates from the inherited style to the custom style and make changes there. However, templates in the inherited style will change over time. A board owner will have to periodically look at the differences in their template against the inherited template and make changes accordingly.
However, if phpBB allowed use of these template events in custom styles outside of an extension, it would address this particular issue since many changes will be tied to template events. It's a common pattern to add code like additional <link> and <script> tags to events to do things like add tracking code not supported by an extension or to add additional links to the navigation bar. I suggest this functionality should be built into phpBB.