Hi, I need to format a data table by making certain cells bold, increasing the font size and, most importantly, using special characters. I made a spreadsheet in Excel with special characters (green check marks, red x's, etc) and larger font sizes, and the whole thing looks great. Once I put it in wpdata tables, it's bland and none of the characters show up. Also, I clicked to hide the "wdt_ID" column but it still shows up. Please help. 

Since its the weekend and not sure the response time, I wanted to provide you some assistance. First off, some good resources on custom CSS, which is what you're asking for here:

stackoverflow (asking specific questions); Mozilla's developer site (general understanding of specific elements) and css-tricks.com (great site for examples).

Question 1. As to your cell question, I'd direct you to this stackoverflow answer. In terms of wpDataTables, I've pasted some syntax below. In the below example, it will make the the font bold in row 1, column 5. 

table.wpDataTable  tbody tr:first-child td:nth-child(5) {font-weight:900 !important;}

This same logic would apply for changing font sizes etc. You would just add that syntax. The caveat to keep in mind is that the above will statically change an individual cell and has nothing to do with what that cell contains. So if you're bolding something based on content for that cell, that would require javascript probably. Lastly, they do offer some conditional formatting options under a column's column settings. But I think that is different from what you are looking for. 

Question 2. In terms of special characters, I would try what is suggested here first. If that doesn't work, perhaps it is a wpDataTable issue related to your data source (I'm not observing the issue on mine, and I use Google Sheets), and the folks at wpDataTables could help.

Question 3. I would try hiding it again and make sure you click both "Apply" and "Save". If you just click "Apply", I don't believe the change will take affect. 

Hope this helps. 

One more thought for the special characters and hidden column- if what I suggested doesn't work, try and flush the Wordpress cache. That's always best practice anyhow.