Can't change borders on my table!

As you have found out ~ borders on category tables cannot be changed. You can change the fill, font and colour though.
Most seem to have 'tear hair out' times with the borders, hope the info below helps.

The Border Options are on the format bar but the Border options in Inspector might be easier to follow.

NOTE: Blue double lines show which borders are selected, they disappear when the borders have been changed.

Inspector > Tables > Cell Borders.
First select the cell / cells in your Table or select the Whole table > select which borders you want to change in the Inspector > Then select the line but remember Thin will not allow you to change the next box which is the point size of the line. > next box is color.

If you just want to select separate lines or Non contiguous borders in the table : Select "Allow border selection" on the Menu bar Table dropdown. and is also under the cell button on the format bar.

Select any cell to select table. Select single border, click again if you want a single cell border Hold down command key and set as many as you wish. Curser is an equals sign with double ended arrow through it. Clicking the border toggles all row / column border or single cell border.
Then set line, width, colour etc.

If you get a mess which we all do at times Select the whole table in the side bar and set the Line option box to None and start again.

Have fun 🙂