[LukeShu]

For some reason, at width <= 480, it sets

    word-break: break-all

which sets the character-splitting to CJK (Chinese/Japanese/Korean), where splitting between characters is OK. This should generally not be used on English text.

Perhaps the author meant to use

    word-wrap: break-word

Which allows it to split a word that is too long to fit on a single line, instead of stretching/extending outside of the text area.

[gruntmaster9000]

The blog theme was made by a Chinese developer, which is probably why that is set the way it is.