"Un étui" is probably best translated as 'a case' in most cases, although in the context of this song 'box' works fine.

As for "légère, légèrement" it can be translated as 'light, lightly', 'soft, softly' or 'gentle, gently'.
In my opinion, its semantic function is not to add any precision to the text or qualify specific parts of it but imbue the whole song with a reverie-like or dreamy-like quality (as can be done in photography w special filters or on stage w fog or flood lights). The first time I ever heard it -in Normandy, as a young adult- it produced a strong esthetical impression (vision-like) on me; of backlit distant silhouettes against 'glittering' golden sands (as can be experienced on the coast of Normandy esp at both the Mont Saint Michel and Somme river bays) slowly growing more identifyable as they approach. When I read the lyrics at some later stage, I realised how little of that 'sands' vision was in the actual words. It seems the words "là-bas" (over-there), "bancs" (sand-bancs) and "légèrement" (light) were enough to imprint a whole Impressionist- or Turner-style scene in my mind's eye.
Where did the light come from? I did not speak English at the time so léger=light could not have suggested it. Could the sole mention of Fécamp have triggered it?