1) As mentioned in a previous topic, you cannot. However, you *can* make your own buttons or download other ones to use. And I have to disagree with your opinion on the icons. I will admit that without a background, they may be a little harder to see, but you can add a nice background and they look just fine. They are also very crisp. Most people who complain about them just haven't gotten used to them yet and within a short period of time no longer notice them being different.
2) Turn off the reload logs option. Alt-O > IRC > Logging.
4) Turn off the Line Marker. Alt-O > Display > Options > Line Marker.
