How many /r/ sounds are there in the word? Listen to the audio file if you are not sure.
darker → ...
We do not pronounce r after vowels.
In a stressed syllable, a vowel + r can have three sounds:
ɜ: bird, surf, herd, learn
ɔ: lord, four
The unstressed er, or, ur become ə (the Schwa sound).
rather, computer, director, sulphur