I hope this amply answers ur question, best of luck.

Role of Customs in Islamic Legislation with special reference to Arabian Customs Adopted by Shari'ah
1- Introduction:
Islamic legal system has its origin in Arabia and has been developed by arab jurists. The groundwork of the Muhammadan legal system, like that of other legal systems, isto be found in th customs and usages of the people among whom it grew and developed. The Muhammadan code includes many rules of Pre-Islamic Customary Law which have been embodied in it by express or implied recognition.

2- Meaning of Custom:
According to Ibn Najeem:
"Those recurring practices which are acceptable to people of sound mind."

3- Custom As Source of Law:
In Islamic la, custom is regarded as a secondary source and Hanafi writers include it as a source of law, under the Principle of Istehsan.

4- Force of Custom:
Customs generally are spoken of as having the force of Ijma, and their validity is based on the same texts as the validity of the later.

(I) Preferred over Analogical Deduction:
Though custom does not command any spiritual authority like Ijma of the learned, but a transaction sanctioned by custom is legally operative, even if it be inviolation of a rule of law derived from analogy. However it must not oppose to Holy Quran or Sunnah.

5- Significance of Pre-Islamic Customs:
Those customs and usages of the people of Arabia which were not expressly repealed during the life time of the Holy Propher P.B.U.H are held to have been sanctioned by the lawgiver, by his silence and retained in the Islamic law.

6- Modes of Recognition of Pre-Islamic Customs in Islam:
There are two modes for the recognition of Pre-Islamic Customs in Islam.

(I) Express:
Some of the Pre-Islamic customs were expressly recognized by Islam.
>>>For example: Islam expressly recognized one of the different modes of marriage prevalent among Arab people in which a man ask another person for the hand of his daughter and then marries her by fixing a dower.
(II) Implied:
Implied recognition of customs means those pre-Islamic customs which were not expressly abrogated by Islam. They were deemed sanctioned by the silence of Holy Prophet (Peace be Upon Him).

7- Reason for Recognition of Pre-Islamic Customs in Islamic Structure:

Following are the reasons of the basis of which the customs are recognized and included in Islamic Law.

(I) Based on Shariat-i-Ibrahimi:
Arab customs which are incorporated in the Islamic law are those which related to Shariat-i-Ibrahimi.
(II) Claim of Islam:
Islam never claimed to be the original religion, some customs were related to other religions which were given to the whole chain of the Prophets.
Holy Prophet (Peace be Upon Him) said, "He has revealed the book to you confirming the preceded scriptures, for he has already revealed Gospel and Toreh, for the guidance of human being."
(III) Reformatory Spirit In Islam:
Because of its reformatory spirit, Islam modified certain laws which needed to be amended and retained those laws or customs which were just.

8- Examples of Some Pre-Islamic Customs which are Adopted by Islam:

(I) Customs of Punishment:
i- Punishment by retaliation commutable to a payment of blood money or compensation for the injury.
>> Punishment of Qisas and Diyat is imposed by Islam.

ii- Imputation of Right hand of thief.
>>This custom was adopted by Islam.

iii- Adulterer to be stoned to death if poor, otherwise blackening his face and flogging him.
>>If the person who commits zina is Mohsin, he is to be stonned to death.

(II) Customs Regarding Marraige:
i- Man asked another for the hand of latter's ward or daughter and then marries her by giving her a dower.
>>Islam recognized this custom.

ii- Husband asked his wife to have Cohabitation with a famous man to secure a noble seed.
>>This custom was abrogated by Islam.

iii- Number of men less than 10 used to have intercourse with a woman.
>>This custom was abrogated by Islam.

iv- Prostitution was common
>>This custom was abrogated by Islam.

(III) Customs of Dower:
In the regular form of marriage the fixing of dower for the benefit of the wife was in vogue among the Pre-Islamic Arab. It formed the marriage contract.
>>Islam recognized this custom as Quran says, "And give women their dower freely."

(IV) Polygamy:
There was no restriction to the number of wives in Arab.
>>Islam does not allow more than 4 wives for a Muslim at a time.

(V) Divorce:
In pre-Islamic Arab, there were four modes of divorce which may be pronounced at any time.
(i) Talaq (ii) Ila (iii) Zihar (iv) Khula
>>Islam disapproved pre-Islamic custom and allowed three distinct period under which spouse can patch up and after lapse of third period, talaq is effected.

(VI) Custom of Succession and Inheritance:
In pre-Islamic Arab, women and minors had no right of succession and inheritance.
>>Islam abolishes this custom and gives women and minors right of succession and inheritance.

9- Conclusion:
To conclude, I can say, that Islam is a religion of nature and it would not be correct to suppose that Islam professed to repeal the entire customary law of Arabia and to replace it with a code altogether new laws. Islam adopts certain customs of pre-Islamic era and certain customs were also abolished by Islam and to discover the rules of Islamic Law and to interpret their meaning, the knowledge of customs prevalent at the time of revelation is essential.

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"A successful person is one who can lay a firm foundation with the bricks that others throw at him." (David Brinkley)