Improve answer options: display semi-transparent ○/× symbols on buttons

[scriptin]

Display circle (○) or cross (×) symbol on SVG buttons with answer options when they are clicked to provide additional indication of the result. Those symbols should fill the entire button and be semi-transparent, so that they do not obstruct kanji or get obstructed.

Currently it's only the green or red color which indicate if the answer was successful or not, which may be problematic for color-blind users.