I have two repos named official and custom. I need to merge the official repo into custom while adding any missing lines from official but keeping any lines from custom that exist but differ.

The goal is to ensures to get all new functionality from the official theme while keeping my customizations intact.

For example

official: file1
new line
conf parameter: 1


custom: file1
conf parameter: 2

After merging the desired result should be

custom: file1

new line
conf parameter: 2

I tried merging in the following ways:

git merge --allow-unrelated-histories --strategy=recursive -X theirs official/master

custom:file1
new line
conf parameter: 1

git merge --allow-unrelated-histories --strategy=recursive -X ours official/master

custom:file1
conf parameter: 2

Is there another way to achieve this than resolving the merge conflict manually ?