How to underline (rather than highlight) the current line in vim?

try :hi clear CursorLine to clear the current cusorline hl, then :hi CursorLine gui=underline cterm=underline

The color of underline is same as your ctermfg or guifg. You can either live with your "colorful" underline, or add cterm/guifg to make the underlined text and the underline same color.

:set cursorline

was the best solution for me, you can combine it with removing the cursorLine as the answer above mentions