Hello…
I am just beginning to work with Tiled and have successfully created a test-bad map for my developing war-game algorithms.
I would like to know how I could darken the sides of hexagonal tiles as I can barely see them in the editor and when I export them to a PNG image,they completely disappear.
Thank you for any suggestions anyone may have… 
If you mean the sides of the tiles in your tileset, you’ll need to edit them in an image editor.
If you mean the tile grid, if you’re exporting with the “Draw tile grid” option, I don’t think you can do anything other than perhaps setting a different grid colour in Edit > Preferences > Interface, but I think the default is already black.
Thank you very much, Eishiya… 
Your suggestion worked.
I changed the outline color to white and the hexagon outlines are now visible…