I discovered a way to do this. LibreOffice Ver: 7.0.3.1 (x64)
Select the cells you want to increase the border width on; right click; select format cells; borders; under "user defined, click on which line you want to modify; then lower down under “line”, choose style (if you want something other than a straight line) and then click on line width (up or down arrow) to choose the weight of line you require.
Hold the CTRL key down to choose more than 1 line and all lines will be affected when the line width in changed.
Hope this helps…