One important detail needs to be mentioned, before your question can be answered correctly.
Thief or Covet (Stealing moves) will fail to steal an item if any one of the following happens:
A. The Pokemon using the stealing move HOLDS IT'S OWN item (was that your issue with Arcanine Siresh'd?)
B. The Pokemon the stealing move hits has Sticky Hold (an ability that prevents item theft/removal)
C. The item is unstealable (Mega Stone or Z Crystal). Life Orb is stealable.
D. If the Pokemon is immune to normal type moves, Covet WILL fail, unless Foresight was used first. Nothing is immune to a dark type, so that moves works as normal.
For the following Example we'll use a fully ev trained Eevee, with a Jolly Nature, and holding NO ITEM versus a Musharna, with a Quiet nature, holding a Life Orb (Eevee will always go first). Game starts at turn 1.
The first turn would be something like this.
A. Game checks Pokemon's speed stats (including any stat changes, Choice Scarf, or Paralysis). As it's turn 1, none of that has happened. Eevee will go first.
B. What moves were chosen? Eevee chose Covet, Musharna chose Calm Mind.
C. Game checks what Eevee's attack stat is, versus Musharna's Defence and does it's internal math to decide how much damage is going to be dealt.
D. The game checks and attempts to factor in (IF applicable) STAB, Super Effective/Resisted/Immunity damage multipliers (x2 per type, x0.5 per type, and x0).
E. The game checks if there is any stat changes to Eevee's attack or Musharna's Defence, then checks if Eevee has an item that affects it's damage with Covet (Normal Gem, Life Orb, Choice Band, Muscle Band, etc).
F: Eevee deals it's damage, and THEN obtains the Life Orb as that's how Thief and Covet work. Can't explain "how" that's in the game's coding.
During steps A through F, Eevee had no Life Orb, it gained the life Orb AFTER Covet dealt damage.
Eevee does not lose 10% of it's HP (rounded down), nor does Eevee deal 50% more damage this turn.
Source:
Experience, I've played since Blue came out on the Original Game Boy.
