Our goal with the basic plan is to make it as simple as possible to create and manage a community on Discourse.
While many communities want to further customize Discourse with things like themes and theme components, there is plenty of configurability already built into Discourse, and the basic plan also includes a number of plugins that we determined would be valuable for small, private communities.
Installing themes and theme components do require some additional understanding and risks as these extensions can be developed by anyone and have to maintain compatibility with Discourse, and there are not currently any limitations built into Discourse to prevent installing things which may not be fully supported.
That said, we are also exploring ways to provide better guidance to admins and theme developers mitigate these risks as we’d like to improve that experience for self hosters and on other plans.
As for the example of category icons, this is the kind of thing that is likely a candidate to become a core feature of Discourse at some point in the future.