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 =: