Theme: Fix blank icons having a background

Theme: Fix blank icons having a background

A fix from 43254468de7d69b5422e667ba7ebbe864fc34a63 introduced a rule setting
`background-image` of a blank icon to none. However, the selector used for that
rule had lower specificity than another one: `.ui-button .ui-icon` which caused
dashes being shown over the icons.

We needed to increase the specificity of the `.ui-icon-blank` rule past the
above selector and past `.ui-button:hover .ui-icon`. We're doing it by repeating
the class name three times.

Closes gh-1987