I think that in addition to Opera's official themes and one customizable theme there should be an option to save, name and select (and possibly share) our custom themes.
There are multiple reason to implement this:
Possibility to experiment new primary/secondary combinations without losing you favourite one;
Quick switch between your custom themes;
Option to have your themes (official or custom) switch automaticaly after the selected number of minutes or in different hour of the day;
Creation of a community that exchange their custom themes (accessible with the forum or with the opera addons);
Possibility to save custom themes not only locally but also on your opera account;
These new function not only will bring new features in our favourite browser but also will result in a creation of new opera accounts (investors, i'm looking at you) since there will be a new functionality to them and will create a community of themes sharer.
Thank you for your attention, hope to see this functionality in the new updates