The border color is hardcoded as
Qt::black. Therefore, if you want to change the color, you'll need to get the source, modify it, and recompile Okular. Look in
pagepainter.cpp starting on line 711 (comment mine):
if ( borderWidth != 0 )
QPen pen( Qt::black, borderWidth ); // The first argument is the important part
painter.setPen( pen );
painter.drawRect( 0, 0, image.width() - 1, image.height() - 1 );
If you want to remove the border, you can just delete the above if statement and its contents. If you want a different color, you can replace
Qt::black with another one of the twenty predefined colors. To get any color, replace that constant with a call to
QColor::fromRgb. For example,
QColor::fromRgb(239, 228, 176, 255) is a kind of dingy yellow.
Recompiling applications is a little out of scope here, and a full explanation would be a tome. Here are some reference documents from KDE TechBase, though they may be a little out of date:
I wish there was another way, and I would welcome a simpler answer.