You can edit your custom theme by here!
Currently MikuMikuWeather indicates daily weather as combination of two weather status. Primary is a main weather of the day, and secondary is supplemental.

Fill all empty areas () with your weather icon image by Drag&Drop or clicking it to select file.

I suggests that images has at least 500x500 pixels. You can upload any image larger than that, however it automatically resized to 500x500 when packaging. (If you allow to download original images, server keeps original for that.)

Bookmark current page or note following URL to continue edit. This draft will be kept 3 days at least from last modification.
(Note: ANYONE known the URL can view/edit this draft.)