Add max-width property to all images?

[ddarfantasy]

Some large images can appear out of frame and look ugly in the new default colors.
I'm not professional on css, but I think it's partly because of the incomplete fluidbox design?
And before it lands, is there any workaround to the problem?

My idea is to simply add this or something equivalent to theme.css:

img {
    max-width: 100%;
}