Minimal(?) example of custom theme in Jetpack Compose

MyTheme.kt

import androidx.compose.desktop.Window
import androidx.compose.material.*
import androidx.compose.runtime.*
import androidx.compose.ui.graphics.Color

fun isSystemInDarkTheme(): Boolean {
    // TODO: On Android, presumably this is `AppCompatDelegate.getDefaultNightMode()`.
    //       What is it on Desktop?
    return false
}

@Composable
fun MyTheme(
    darkTheme: Boolean = isSystemInDarkTheme(),
    content: @Composable () -> Unit
) {
    // The only two values I actually have in the brand
    val desire = Color(0xe8, 0x37, 0x55)
    val tuftsBlue = Color(0x41, 0x98, 0xdf)

    // The rest I still have to provide for some reason, presumably Compose
    // is too lazy to apply whatever function is necessary to darken or lighten
    // a colour slightly?
    val primaryColor = desire
    val primaryLightColor = Color(0xff, 0x6f, 0x81)
    val primaryDarkColor = Color(0xaf, 0x00, 0x2d)
    val secondaryColor = tuftsBlue
    val secondaryLightColor = Color(0x7c, 0xc8, 0xff)
    val secondaryDarkColor = Color(0x00, 0x6a, 0xad)

    // Also note how we have to figure out for ourselves how the terminology
    // in the theme creation tools maps to the terminology in the APIs.
    val colors = if (darkTheme) {
        darkColors(
            primary = primaryColor,
            primaryVariant = primaryLightColor,
            secondary = secondaryColor,
            secondaryVariant = secondaryLightColor
        )
    } else {
        lightColors(
            primary = primaryColor,
            primaryVariant = primaryDarkColor,
            secondary = secondaryColor,
            secondaryVariant = secondaryDarkColor
        )
    }

    MaterialTheme(colors = colors) {
        content()
    }
}