Just about everything in a Core Plot bar graph is customizable, from border color to line width to fill color (or gradient). The screenshots on the project site are merely some defaults that are used in the sample applications that come with the framework.
If you look at the long list of applications that use Core Plot, you can see many different styles of graphs, including some different takes on bar charts.
There are a few stock themes, which can be used to create a default graph look using code like the following:
// Create barChart from theme
barChart = [[CPTXYGraph alloc] initWithFrame:CGRectZero];
CPTTheme *theme = [CPTTheme themeNamed:kCPTDarkGradientTheme];
[barChart applyTheme:theme];
Built-in themes currently include kCPTDarkGradientTheme
, kCPTPlainBlackTheme
, kCPTPlainWhiteTheme
, kCPTSlateTheme
, and kCPTStocksTheme
, but you can create your own or use these as a starting point to customize your bar chart however you like.
If you can't figure out how to do a particular style of bar chart using Core Plot, you can ask about that on the mailing list or in a focused question here.