Regarding to config, I've pasted what is defined in the readme's setup.
highlights.scm found.locals.scm found.textobjects.scm query found for javasublimemonokaiTSBufEnable highlight
TSBufDisable highlight
The differences are:
Main from non-italic to italic.void from non-italic to italic.String from non-italic to italic.main.defaultTSBufEnable highlight
TSBufDisable highlight
The differences are:
Main.static.void.main.String[].args.String and variable name element.The highlights for argument is TSParameter you can configure it freely and locals currently have no highlight.
Some symbols may not be highlighted after enabling the feature on the fly, you can try forcing the buffer to reload :e! or redraw (Ctrl-L), or just give it a try by enabling it by default :) this is how a java file looks like for me

You can modify the highlight groups defined in https://github.com/nvim-treesitter/nvim-treesitter/blob/master/plugin/nvim-treesitter.vim#L25-L75 as you like (like marking some elements with italics).
I've tried a few but not all of them work.
hi TSMethod guifg=#0066FF " Only one working
hi TSParameter guifg=red
hi TSProperty guifg=pink
hi TSField guifg=forestgreen
hi TSConstructor guifg=#66FF22
This is the result:

Is there something I'm missing?
I've looked into the field groups that you are trying to use and it looks like they don't necessarily line up.
TSMethod -> works.
TSParameter -> works.
TSProperty -> not used.
TSField -> not used, seems to be mapped TSType instead.
TSConstructor -> not used, also mapped to TSType.
We can update the queries to better match the expected highlight groups, or at least add documentation so that users know which to use.
@vigoux should we close this in favor of #81 ?
Is there something I'm missing?
surely, because atleast TSParameter works for me (java)