You can assign custom keyboard shortcuts in the section Keystrokes of the preferences dialog, accessible from menu Tools > Preferences.
To assign keyboard shortcuts that do not use modifiers such as control and alt, it is useful that you disable editing of a node upon typing outside of an editor. This you can do in the preferences dialog (menu Tools > Preferences), in the section Behavior > Key Typing, by checking the check box "Disable Key Type". This makes it possible to assign keyboard shortcuts on the model of the text editor Vim. In addition, in the preferences dialog, you have to enter the single letters capitalized by holding shift, so "B" instead of "b". And yet, assignment of "B" instead of "control + B" to boldface leads to a buggy behavior.