In MLT there isn't really a role for a dedicated Thief class. There's no dungeons, few traps and locks aren't all that complicated in the main.
So I've taken two steps. Firstly, codified/collected the "derived throws" (it needs a better name) that are referenced elsewhere. Everyone can do these:
Brute Force - to open stuck doors or any other feats of raw strength requires a proficiency throw of 18+. Particularly difficult tasks may impose a penalty on this proficiency throw. In addition, for each point of Strength adjustment, modify the result of the die roll by +/- 4. (A character with Strength 18 thus opens doors with a proficiency throw of 6+). If two characters cooperate to apply their muscle, use the stronger character’s Strength adjustment +4. A roll of 1 always fails to achieve the goal.
Climb Walls - any able-bodied character can scale steep surfaces such walls or cliffs without equipment as long as there are ample hand-holds. Sheer surfaces and those without require the appropriate equipment to attempt. They require a proficiency throw of 14+ (modified by Strength adjustment) for each 100' they intend to climb. If the roll fails, they fall a distance equal to half the attempted distance, plus the distance covered by any previous throws, taking 1d6 points of damage per 10 feet. Characters climb at ¼ their standard combat movement rate.
Hear Noise - characters can can attempt to listen for noises in a cave or hallway and at a door or other locations. The character must be quiet and in a quiet environment. They require a proficiency throw of 14+ (modified by Wisdom adjustment) to pick up enough to understand what they are hearing. Hear Noise can be used for general perception and observation checks, whether passively noticing clues or actively searching for them.
Sneak - All characters can attempt to hide in shadows and move silently. This requires a proficiency throw of 18+ modified by their Dexterity adjustment. A character may attempt to skulk unseen in the cover of darkness.They will always think they are successful in this skill, and will not know otherwise until others react to their presence. A character will remain hidden so long as they stay motionless. If they move, they must make a new proficiency throw to hide.
Characters may move with total silence. When successful, even keen eared guards will not hear their movements. However, they always thinks they are successful in this skill, and will not know otherwise unless and until others react to their presence. Characters may move silently at 1/4 their standard combat movement rate without penalty. If they move greater than 1/4 speed, they take a -5 penalty to the proficiency throw. If they move at normal speed or faster, they take a -10 penalty. Wearing armour imposes a -2 penalty on attempts to move silently, this rises to -4 if wearing metal armour.
Secondly, I've collated the Proficiency alterations that capture them all:
Alteration: Alertness - The character's base proficiency throw for Hear Noise becomes 14+ and improves by one point every level. He gains a +1 bonus to avoid surprise.
Alteration: Climbing- The character can climb cliffs, branchless trees, walls, and other sheer surfaces, without climbing aids. In addition, his base proficiency throw for Climb Walls is 6+.
Alteration: Dungeon Bashing - renamed Brawn Over Brains - The character is hardened to heavy lifting and physical labor. He receives a +4 bonus on throws to open doors and similar acts of brute strength.
Deletion: Eavesdropping - this proficiency is removed.
Alteration: Skulking - The character excels at moving furtively and finding concealment. His base proficiency throw for Sneak is now 14+ improving by one point every level. His penalty to moving silently while wearing armour is reduced by 2 points. He may also move at 1/2 standard combat move without penalty. If he moves greater than ½ speed, he take a -5 penalty to the proficiency throw. If he runs, he take a -10 penalty.
New Proficiency: Disable Device - The character is skilled at disabling locks, traps and other devices. This requires a base proficiency throw of 18+ modified by their Dexterity adjustment, which improves by one point every level. With the aid of thieves’ tools, the character may pick mechanical locks. He may only try to pick a particular lock once, and if he fails, he may not try the same lock again until he reaches a higher experience level. The GM may apply bonuses or penalties to the roll, depending on the complexity of the lock.
Through careful inspection and probing, the character may find hidden traps and then attempt to disable or discharge the trap harmlessly. He may only try to find or remove a trap once in any given area. The GM may apply bonuses or penalties to the roll, depending on the complexity of the trap. If the character fails, he may not try again until he reaches a higher experience level. Note that these are separate skills, for a character must find a trap before he can remove it.
New Proficiency: Pick Pockets - The character may relieve an unaware target of items on their person such as pouches, jewelry, small items in pockets and so on. This requires a base proficiency throw of 14+, modified by Dexterity, which improves by one point every level. There is also a -1 penalty on the proficiency throw per each level the character is lower than the victim. A throw that is less than half the target value means that the intended target notices the thieving attempt. The GM will then make a reaction roll with a -3 penalty to determine the intended victim’s reaction. It may also be used for sleight of hand tricks suitable for impressing the guillible and to palm and conceal small items. When performing sleight of hand in front of an audience, the character may also apply your Charisma adjustment to the roll.