Option: Versatile Banner theme component
Probably the easiest way to impliment this would be using this theme component. Which also allows other customisations for when it will be displayed i.e. homepage only, anonymous users only etc.
Option: Simple HTML/CSS theme component (site-wide)
If you just want a simple site-wide banner, this would likely be the most lightweight:
- Go to admin > customise > themes > components > install > + create new.
- Input name e.g. ‘site-wide banner’ select type ‘component’.
- Edit css/html of the new component, select the common > after header and input your banner html/css. You can optionally specify different banners for mobile and desktop if you like also, just select mobile or desktop instead of common.
- Add the component to your currently used theme to activate it.