`const val` not being used in au.com.shiftyjelly.pocketcasts.ui.theme.ThemeColor.kt

### Description

I realized while doing some memory profiling that ThemeColor.kt uses a fairly big amount of memory for what is essentially a constants class. I believe a slight reduction in memory use could be obtained from prepending `const` to all the colors defined in ThemeColor.kt

### Step-by-step reproduction instructions

1. Open up Pocket Casts on Android Studio with profiling with complete data
2. Capture a heap dump
3. Observe that ThemeColor.kt will be one of the objects with the highest memory use

### Screenshots or screen recording


### Did you search for existing bug reports?

- [X] I have searched for existing bug reports.

### Device, Operating system, and Pocket Casts app version

API level 34, Linux. Tested on Pixel 7 Pro emulator and a real Samsung S21

Open ThemeColor.kt and inspect the color definitions that currently lack const. Apply the requested change to all applicable colors, then use the Android Studio memory profiler and a heap dump to check whether ThemeColor.kt uses less memory.

Written by the indexing model from the issue text.