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The CSS given on that old thread you revived was for the regular Matrix Field (not for the Multiselect Grid Widget).
Also, the CSS works as intended too. Here's a test form showing a styled Matrix Field using those codes given by my colleague https://www.jotform.com/70594143344960
Here's the complete snippet again in case you need it http://pastebin.com/raw/UD61B7td
Related guide: How-to-Inject-Custom-CSS-Codes