Theocritus appears to have created, from scratch, a literary dialect that borrows from several Doric dialects. A Sicilian, he would have spoken some Doric dialect natively, but it isn't clear that all the doricisms we find in his poems occurred together in a single dialect. In this he followed the Epic dialect of Homer which borrows grammatical features and vocabulary from a number of dialects, which gives flexibility in the dactylic hexameter and also serves to distance poetic language from everyday speech.
Like the choral Doric, this Theocritan bucolic dialect is a Doric patina that colors an Epic dialect base. For example, second declension singular genitives may be found in ‐οιο, and dative plurals in ‐οισι. Like Homer, Theocritus borrowed some Aeolic forms into his poetic language. Finally, although Theocritus used the dactylic hexameter he generally avoided Epic diction, using mostly Koine and occasionally Doric vocabulary.
We don't really know how Theocritus' Alexandrian audiences would have felt about the Doric coloring in his poetry. Some have suggested it would have sounded countrified or maybe a bit hick-ish, but that's not at all clear. There's no reason to think cow herding was an especially Doric vocation. Also, though later bucoliasts were very bucolic, the poems of Theocritus cover a wide range of subjects.
Finally, the manuscript tradition for Theocritus presents a dialect mess. One source will use a doricism in a word where other sources have an Epic or Koine form, but will then go on to have a Epic form where the others have Doric. We know early editors liked to force texts into the suitable dialect for some authors, and not always very skillfully.
Theocritus' followers - Bion, Moschus and a few anonymous poets - adopted this literary dialect for their own work.
Original long alpha is retained, as in the Aeolic dialect. e.g. ἁμῖν = ἡμῖν.
In verbs ‐εο‐ contracts to ‐ευ‐. Or, it may resist contraction, as in Epic. In the course of two lines of Bion 9, we see both φοβέονται and φιλεῦντι.
Sometimes ‐σδ‐ for ‐ζ‐.
In the temporal adverbs, ‐κα instead of ‐τε, e.g. ποκα = ποτε. Sometimes the kappa is doubled, as in ὅκκα for ὅτε.
First Declension: long alpha for eta; accusative plural ‐ας may be long or short; genitive plural in ‐ᾶν. Masculine first declensions in -ας (Attic ‐ης/-ας) take the genitive singular -α (long, < ‐αο).
Second declension: genitive singular in ‐ω (τῶ = τοῦ); accusative plural in ‐ως (note accent: τώς = τούς).
3rd person singular accusative pronoun may be μιν as in Epic, or νιν.
2nd person singular forms vary: σύ/τύ, σέ/τέ, τευ/τεῦς/σου, dative τιν/τοι.
1st person plural active ending in ‐μες rather than ‐μεν, e.g. ἐδοκεῦμες. Occasionally 2nd singular in ‐ες rather than ‐εις. 3rd plural ‐οντι for ‐ουσι.
Sometimes infinitive in ‐εν for ‐ειν.
Infinitive εἶμεν = εἶναι, "to be."
Feminine participle ‐οισα for ‐ουσα, an Aeolic borrowing (papyrus evidence hints this is at least in part the work of creative copyists). For ‐εω verbs it contracts to ‐εῦσα.
Future of ‐ζω in ‐ξῶ rather than ‐σω. All futures contracted (‐ῶ for ‐ω).
In ‐μι verbs 3.sg. in ‐τι rather than ‐σι; and 3.pl. in ‐αντι/‐εντι/‐οντι for ‐ασι.
κᾱ = Epic κε, Attic ἄν. ἤν for ἐάν. κἤν for κἄν.
αἰ for εἰ.
ποτί, ποτ for πρός.
τηνεῖ = ἐκεῖ; τῆνος = ἐκεῖνος; ἁμᾷ = ὁμοῦ.