This sentence pattern is somewhat archaic, or at least stilted and formal, in Modern English.
In older stages of the language, a relatively common way of expressing the notion of phrase-initial “whether [it is] A (or B)” and “if [it is] A” was to use the subjunctive of the verb in the appropriate person.
Whether they are red or white, I like roses.
If he is not there, I shall look for him elsewhere.
– could both be rephrased as:
Be they red or white, I like roses.
Be he not there, I shall look for him elsewhere.
In both cases, ‘be’ is the third person singular subjunctive of the verb ‘to be’ (which happens to be identical to the imperative and the infinitive—but that’s just a quirk of English morphology).
A similar expression introduces the word ‘ever so’ to these phrases: “be X ever so Y” then means, “however Y X is/may be”:
I do like Gucci bags, be they ever so overpriced.
– is the same as:
I do like Gucci bags, however overpriced they are/may be.
(That is a lie, by the way; I hate Gucci bags.)
This latter construction is especially common in folk songs, where ‘ever’ is often contracted to ‘e’er’.
As mentioned above, this construction has become somewhat unusual in the current language, but it can still be employed as a stylistic variant. The construction is limited to the verb ‘to be’, however—if you use it with any other verb, you are quite likely to draw quite a few blank stares and odd looks.