MDL-37771 fix questionbank display in some themes.

Some themes (binarius, boxxie) were applying a style to
.block .header { ... }
This will break anywhere where a table is included in a block, so I changed this to
.block div.header { ... }
(The .header class is used for both bits of blocks and bits of tables.)

Others (anomaly, splash) applies a style to all th in the question bank. That breaks because
recently one column was changed to th for accessibility reasons, so I changed that rule to
th.header.