Is there a way to change the code app black background color to be equal to terminal color to get a cool integration? Look at the image for a better understanding.
Short answer: edit
/usr/share/gtksourceview-3.0/styles/solarized-dark.xml and customize it the way you want. For example, line 29, changing the value of 'base3' from #002B36 to #2B2B2B will get you the desired background color.
Long answer: take a look at Dark theme on Elemntary Code instead of “Solar Dark”
If you mean the Terminal's background, then you need to install the Elementary Tweaks and go to Terminal menu —> Background Color and change the background color, you can also change the background transparency.
Also, I don't recommend changing the background color from gray to any other color or setting the transparency too high. In my opinion it doesn't look neat, but you can do whatever you want!