Currently there is no ability to format a table that has been inserted into the document. Specifically there is no method to change the cell borders in any way.
That's not true. There has always been the Table > Autoformat feature [1] that was enhanced one of the last year's GSoC projects. You can now apply a style to your tables [2,3] (actually not a real style but working similarly) via sidebar Styles and Formatting > Table Styles. Modifying this style changes the appearance of all tables in your document. Some pieces are still missing, in particular a GUI to edit the table styles. You will easily find the respective tickets. [1] https://help.libreoffice.org/Writer/AutoFormat_for_Tables [2] https://design.blog.documentfoundation.org/2015/12/13/style-your-tables/ [3] https://gist.github.com/ubap/55d22ef9b2e00347a2dc58ca4cb8b0ea