in jquery read a multidimensional array via wp_localize_script()

Accessing the data

var data0A = theme_metadata[0].dataA,
    data0B = theme_metadata[0].dataB,
    data1A = theme_metadata[1].dataA,
    data1B = theme_metadata[1].dataB;

[Update] Iterating over numeric arrays in JS
While the initial question was still somewhat pertinent to WordPress, your follow-up really is a question purely concerning javascript syntax. Anyway, this is how:

var dataA,
    dataB;

for ( var i = 0; i < theme_metadata.length; i++ ) {
    dataA = theme_metadata[i].dataA;
    dataB = theme_metadata[i].dataB;
    // do something with the data
}