Greek letters and symbols

You need θ or σ. There are three ways to get it: click it on the Math tab, type its English name, or hold Alt and press a letter. A Greek letter is an ordinary name. σ can be a variable the same way s can.

From the Math tab

The Symbols group puts α β γ δ θ λ μ σ φ ω one click away. Clicking one drops it into the math region that has focus, or opens a new region if nothing is focused. Symbols ▾ goes wider, with categories for α lowercase, Α Uppercase, ∑ Math, $ Money, Hebrew, and Tags.

Next along sits the Constants group: π, e, ∞, and g, which is standard gravity at 9.80665 m/s². Those four are values rather than plain letters, so they arrive with a number attached. A := of your own on any of those names still overwrites the built-in from that region down.

Type the name

Type theta, sigma, or phi and the field renders the letter as you finish the word. Capitalised names work the same way: Delta gives Δ and Sigma gives Σ. Once you know the name this beats opening a picker, and it keeps your hands where they were.

Alt shortcuts

Hold Alt — Option on a Mac — and press a letter. Add Shift for the capital, so Alt+Shift+D gives Δ. The mapping follows the familiar Roman-to-Greek table, so the ten letters on the ribbon grid are:

Shortcut	Letter	Shortcut	Letter
Alt+A	α	Alt+L	λ
Alt+B	β	Alt+M	μ
Alt+G	γ	Alt+S	σ
Alt+D	δ	Alt+F	φ
Alt+Q	θ	Alt+W	ω

The rest of the alphabet follows suit. Alt+P for π, Alt+R for ρ, Alt+T for τ, Alt+C for χ, Alt+E for ε, Alt+H for η, and so on. Only j, o, and v are unmapped, having no natural Greek counterpart.

Subscripts

An underscore after a name starts a subscript, so sigma_x renders as σ with a subscript x. The order does not matter: insert the Greek letter from the ribbon first and type _x after it if that is quicker.

The complete Alt map lives on the keyboard shortcuts page. Note that autocomplete will not finish a Greek name for you. It handles variables and units only.