Those curtains are faded, old fashioned, scruffy or boring. What can you do?
Try machine dyeing them a darker shade or the same shade again. Use two boxes of dye per curtain.
If you are prepared to pay a little more, then cover them with new light-weight, ready-made un-lined curtains.
Buy the same colour.
Try a paler or darker version.
Use a block of another coloured curtain to place in the middle of each curtain side or use a toning pattern.
I buy two light-weight curtains per side, unless the window is narrow. Alternate the loops of the new curtains with the loops of the old ones. End each side of the old curtain with a loop from the light weight curtain. They will cover the curtains completely. However, the fullness and light blocking of the old curtains will remain.