The library provides the wrong feature (`arbitools-mode'). It should
instead provide the feature which matches the filename (but of course
without the ".el" suffix): `arbitools'.
Also there is a space missing in ";;;Commentary:".
And you might want to consider not putting closing parens in the wrong
place. You should fix that at least for the definition of
`arbitools-mode'.
For data, like the value of `arbitools-highlights', which is likely to
be extended in the future it is okay to put closing parens on a separte
line. But you should put both closing parents on the same line and
indent that line correctly. Actually you should reindent the complete
value. But first you need to correctly comment the commented lines,
replacing ";(" with ";; (" (also in other definitions). Then mark the
`defvar' and use `M-x indent-region RET'.