What I did was open the inspector in a new window (undocked), then press Ctrl+Shift+i (Cmd+Alt+i on Mac, I think) to open up
another inspector to inspect the first inspector (!). I then tweaked the styles of the first inspector until I got the layout I wanted. I then saved those altered styles into Custom.css (which updates Chrome's interface automatically - no restart required) and that was it.
It's easier to use the inspector to find the correct styles to alter, because that way you can be certain which styles are being applied in real time, rather than attempting to interpret a CSS file which may or may not include the relevant styles.