You ask if Al-Islam has black-and-white thinking. About some matters, yes, the Islamic ruling is pretty clear cut, but, as you should know, life is not always a black-and-white experience. There are plenty of examples I could cite but I'll stay on point with the particular topic that you raised.
If someone hears the basic message of Al-Islam, understands it, and then rejects to embrace it then on the Day of Judgment, Allah is not going to give any of their good deeds any weight. That is not just a Shia teaching; it is a QUR'ANIC teaching. See, e.g., Surat-ul-Kahf (18), ayah 103-106.
That is a black-and-white ruling from Allah Himself.
However, after a person embraces the Islamic message and way of life, what determines how much ajr (reward) he gets -- that is a multifaceted matter that does not lend itself to black-and-white arithmetic.
In the Qur'an, Allah tells us about something that He calls the Darajat, or "the ranks and degrees", that basically means that we are not all on the same level as Muslims. Some Muslims are more serious about Islam than others. Some are more loyal, more willing to make sacrifices for it, more diligent in keeping up with prayers and other worships, more certain in their belief, more desiring of a spiritual experience (a love affair) with Allah -- and more. All of these factors (and others) distinguish us from one another in the Sight of Allah, and He will reward us for our deeds based on all of these things taken together, PLUS His Rah'mat and Ni'mat (Grace and Favor).
"It is He (Allah) Who has made you His Khalifahs on Earth, and He has raised some of you above others by ranks and degrees, that He may try you in the gifts He has given you. For your Lord is quick in punishment, yet He is indeed Oft-Forgiving, Most Merciful." Surat-ul-An'am (6), ayat 165.
"Not equal are the Believers who sit at home and receive no hurt, and those who strive and fight in the Cause of Allah with their goods and their persons. Allah has granted a grade higher to those who strive and fight with their goods and their persons than to those who sit idle. UNTO ALL HAS ALLAH PROMISED GOOD, BUT THOSE WHO STRIVE AND FIGHT HE HAS DISTINGUISHED ABOVE THOSE WHO SIT IDLE BY A SPECIAL REWARD -- RANKS AND DEGREES SPECIALLY BESTOWED BY HIM, AND FORGIVENESS AND MERCY. FOR ALLAH IS OFT-FORGIVING, MOST MERCIFUL." Surat-an-Nisaa (4), ayah 95-96.
That last quote highlights just one of the things that brings about elevation in the Darajat. There are many others.
Fasting regularly, rising by night and performing salat, showing kindnesses to anyone in need or distress, showing beautiful patience at times of your own distress, HAVING PURE MOTIVES FOR YOUR GOOD DEEDS -- all of that helps and matters.
What we know is that a person who is highly developed as a devotee of Allah get rewarded way more for deeds that appear to be equal or even lesser than deeds done by a less developed person.
I cannot cite a reference to this Hadith, but it well known. I invite anyone who can give you an exact cite to do so. Towards the end of his mission, after the conquest of Mecca, a newly converted Meccan was speaking rudely to one of the Prophet's (alaihi salaam) Companions (R.A.). The Prophet (A.S.) told that Meccan: "Be careful how you talk to my Companions, for you are not his equal. A crust of bread given in charity by him is more precious in the Sight of Allah than an entire mountain of gold given by you."
Brother, in Al-Islam, what really matters is purity. "By the soul and the proportion and order given to it, and its enlightenment as to its wrong and its right -- VERILY, HE SHALL SUCCEED THAT PURIFIES IT, AND HE FAILS THAT CORRUPTS IT." Surat-ush-Shams (91), ayah 7-10.
We must first purify our minds and hearts of false ideas about Allah. Then we must purify our relationship with our Lord. Then we must purify our relationship with Deen-ul-Islam (meaning we need to learn it correctly and then practice it intelligently and diligently). Then we must do all that we can to tune in and fall in love with Allah, and then we must practice our Islam from that place of love.
That is how we purify ourselves, elevate through the Darajat, and thereby increase our ajr earning power in preparation for the Day of Judgment. Allah knows best.
"And the Jannah will be brought near to the righteous -- no more a.thing distant. (A voice will say:)'This is what was promised for you -- for everyone who turned in repentance and guarded (his Faith), who feared the Most Gracious One unseen, and brought to Him a hearted turned in devotion. Enter there in peace and security. This is the Day of Eternal Life!' There will be for them therein all that they wish -- and more besides in our presence." Surat-ul-Qaaf (50), ayah 31-35.
I hope you find this answer helpful. Maa'as'salaam, brother.