Some people, including myself, don’t like the over- and underlining of headlines or sourceblocks defined by leuven-theme. Over- and underlining can be removed easyly. You only have to overwrite the matching attributes after loading the theme, this can be achieved with the set-face-attribute function.

(set-face-attribute 'org-level-1 nil :overline nil) 
(set-face-attribute 'org-level-2 nil :overline nil) 
(set-face-attribute 'org-block-begin-line nil :underline nil)
(set-face-attribute 'org-block-end-line nil :overline nil)

I use use-package to load emacs packages, here is the complete code to load und customize leuven-theme

(use-package leuven-theme
  :if (window-system)
  :ensure t
  
  :init 
  (setq leuven-scale-outline-headlines nil)
  (setq org-fontify-whole-heading-line t)
          
  :config 
  (set-face-attribute 'org-level-1 nil :overline nil) ; I don't like the overline in L1 and L2
  (set-face-attribute 'org-level-2 nil :overline nil) ;  Headings, so I remove it
  (set-face-attribute 'org-block-begin-line nil :underline nil)
  (set-face-attribute 'org-block-end-line nil :overline nil)
)