Move summary screen and credits page Tab button styles to the Modern theme.

Expect that future tab pages reuse these themes.
Fix the selected tab button style of the credits page.
The last line of the former ForegroundBox overwrote the previous three lines, which made the button appear with white text on light-golden background.
Delete now empty theme files for the time being.

Differential Revision: https://code.wildfiregames.com/D786
Approach agreed with: Vladislav, bb