I'm a web dev, not a "writer", so feel free to take my advice with a grain of salt.
Besides the word "sleek", you use "the" to begin three out of 4 sentences in your example. This is also considered bad form and something to be avoided as much as possible.
Sometimes simply changing the word order or adding something small, but useful, can help.
Our trio had come to a halt in front of a large, sleek building. It appeared to be three stories tall with a bridge lined with windows stretching between the two sections. All the walls were smooth and dark. The commander led them through a pair of sleek doors and up the stairs to a room with about a half a dozen people in it.
I didn't change anything too drastic here, but I made it sound a little less like a technical document. I even left in one of the "the"s, since I wasn't sure who the commander referenced was. I assumed that the trio was of the main characters. I also made this paragraph start with all "I"s, with some others sprinkled around, to help show how annoying it can be.
Ok, now for the rewrite of that paragraph to correct myself:
While I didn't change anything too drastic here, it sounds a little less like a technical document. One of the "the"s was left in, since I wasn't sure who the commander referenced was. The trio was assumed to be the main characters. Also, this paragraph starts with all "I"s, with some others sprinkled around, to help show how annoying it can be.
The last sentence is now incorrect, but the paragraph, as a whole, sounds much better. And, yes, I do this in all my writing. Emails, technical documents, and SE answers included. In fact, I wasn't going to do the rewrite, but my OCD required me to make the changes. After a few minutes of consideration, I compromised between giving a bad example and correcting the issues.