Theme: M C. The theme is two words; first begins with M and the second begins with C.
17A. C-E-G triad, e.g. : MAJOR CHORD
25A. 20th-century Greek-American soprano : MARIA CALLAS
35A. Asian language spoken by nearly a billion people : MANDARIN CHINESE
47A. Aladdin's transport : MAGIC CARPET
56A. Rochester medical center : MAYO CLINIC
62A. Toastmaster, and a homophonic hint to this puzzle's five longest answers : EMCEE
Argyle here. I wonder if MAC was the original theme but didn't work out.
Across:
1. Garbage : TRASH
6. Ziploc bag feature : SEAL
10. Catcher's glove : MITT
14. "Slumdog Millionaire" country : INDIA
15. Electrically flexible : AC/DC
16. Not-so-nice smell : ODOR
19. Bandleader Arnaz : DESI
20. Certain Himalayan : NEPALI
21. __ Corps : PEACE
22. High dudgeon : IRE. Haven't seen this old timer in awhile.
27. Singer Etheridge : MELISSA
29. Little fruit pie : TART
30. Prayer ender : AMEN
31. Commercial suffix with Sun and Star : KIST. OJ and tuna.
32. __ Angeles : LOS
40. Place for a mani-pedi : SPA
41. Trent of politics : LOTT. Mississippian.
42. Make a soufflé : BAKE
43. Accompanied by : WITH
44. "Get lost!" : "AMSCRAY!". (Pig Latin)
51. FDR agency : NRA. (National Recovery Administration)
52. Make up (for) : ATONE
53. Orange veggie : CARROT
55. Allow to borrow : LEND
60. Starbucks tea brand : TAZO. Any partakers?
61. Unknown author: Abbr. : ANON
63. Load in a hold : STOW
64. Nine-digit IDs : SSNs. (Social Security Number)
65. Uncool group : NERDS
Down:
1. Buzz Lightyear voice actor __ Allen : TIM
2. Genetic material : RNA
3. Wd. modifying a noun : ADJective
4. Priory of __: "The Da Vinci Code" conspirators : SION. French fraternal organization.
5. Women-only residences : HAREMs
6. Nearly one-third of Africa : SAHARA
7. Often harmful bacteria : E-COLI
8. Sea between Italy and Albania : ADRIATIC
9. Calculator image, for short : LCD. (liquid-crystal display)
10. Ford made only in black from 1914-1925 : MODEL T
11. Perfect : IDEAL
12. Puccini opera : TOSCA
13. Makes an effort : TRIES
18. IRS pros : CPAs. (Certified Public Accountant)
21. Trilogy's first section : PART I
22. Mosque leaders : IMAMs
23. Chart anew : REMAP
24. Kagan of the Supreme Court : ELENA
26. Bills and coins : CASH
28. Not Rep. or Dem. : INDependent
31. Only chess piece that can jump others: Abbr. : KNT. (Knight)
32. Hear (of) : LEARN
33. Schindler of "Schindler's List" : OSKAR
34. "Ta-ta" : "SEE YA"
36. "Through the Looking-Glass" girl : ALICE
37. Univ. military org. : ROTC
38. Cornell University townies : ITHACANS
39. "The Blacklist" network : NBC
43. Place for a pane : WINDOW
44. Cooks' splatter protectors : APRONS
45. Sable automaker, briefly : MERC. Gone the way of the Model T now.
46. Pilfered : STOLEN
47. Fountain treats : MALTS
48. Really got to : ATE AT
49. Purple-blue Muppet with a hooked nose : GONZO
50. Silky synthetic : RAYON
54. "Person of the Year" magazine : TIME
56. Bell and Barker : MAs
57. ATM maker : NCR
58. Mil. roadside hazard : IED. (improvised explosive device)
59. These, in France : CES
Argyle