When you use more than one adjective to designate a single color (like "light blue," "dark green," "pale pink" etc.), neither of the adjectives changes according to the noun it modifies. For example:
Il a les yeux bleu clair et les cheveux brun foncé.
"He has light blue eyes and dark brown hair."
If this sentence were "I wear light blue jeans" it would be worded:
Je porte des jeans bleu clair.
But as the given sentence uses a "verb of appreciation", in this case aimer, then des changes to les.
Just as des does not require translation in my example sentence, nor does les in the given sentence.
If you wanted to specify a particular pair of jeans, you could simply say J'aime le jean bleu clair → "I like the light blue jeans.
But we are speaking of jeans in general here → les jeans plural.
There is also the likelihood that you would only use such specificity if asked. For example, you are in a store:
Q: Quels jeans aimes-tu ? A: Ceux-là. Les bleu clair
Q: "Which jeans do you like?" A: "Those. The light blue ones".
It would be nice if one of the Mods could double check my reasoning here.
Just a question as we often refer to jeans as bluejeans. Does that happen in France? If so, how would you say I prefer light blue bluejeans? In a similar vein, jeans come in various materials in terms of thickness and weight. So (to carry this to an extreme) how would you express the difference? I prefer light (as opposed to heavy) light blue (as opposed to dark, navy etc ) bluejeans.