BorderLiner extension provides a simple method to draw borders in tables and some text elements in LibreOffice Calc and Writer.
Select the line style in the toolbar, use the dialog to define the color. Then activate the extension (left most button). Now imagine the numerical key pad of your keyboard to be a table. "2" marks the bottom border, "8" the top and so on.
Select the cells, press ALT+2 to draw a bottom line, CTRL+2 to pick the linestyle of the bottom line or SHIFT+2 to delete the bottom line etc.