Example:

  * Step:1 executing emacs  -Q in a terminal.


executing emacs -Q in a terminal.
the space in buffer without characters is slightly grey.

  * Step 2: M-: (set-face-background 'default "#303030")


M-: (set-face-background 'default "#303030")

the space in buffer without characters is still grey.

  * Step 3: M-x customize-face RET default RET


M-x customize-face RET default RET
background was set to #303030

  * Step 4: switching to /*scratch*/, putting the cursor to text of
    which background is "#303030", then C-u C-x =:



switching to *scratch*, putting the cursor to text of which background
is "#303030", then C-u C-x =:

Step 5: switch to a end of line, then C-u C-x =:

5. switch to a end of line, then C-u C-x =: