While working on my theme I came across a few things that could easily be fixed from Reaper's Default theme, and some others things I noted.
Missing Images
Few images have been missing for ages now and Reaper seems to be using the built-in images from Reaper 1.0 instead. Which is weird given that they have a working image attribution and it's referenced on the Walter SDK.
item_pooled.png
item_pooled_on.png
midi_inline_fold_custom_view.png
150%/200% files
Some 150% & 200% files are not present while they could →
The _hidpi versions are missing from the Default 6.0 release for all the item_ images.
Code:
item_env_off (sliced-button): (also supports 2x resolution version with _hidpi suffix)
item_env_on (sliced-button): (also supports 2x resolution version with _hidpi suffix)
item_fx_off (sliced-button): (also supports 2x resolution version with _hidpi suffix)
item_fx_on (sliced-button): (also supports 2x resolution version with _hidpi suffix)
item_group (sliced-button): (also supports 2x resolution version with _hidpi suffix)
item_group_sel (sliced-button): (also supports 2x resolution version with _hidpi suffix)
item_lock_off (sliced-button): (also supports 2x resolution version with _hidpi suffix)
item_lock_on (sliced-button): (also supports 2x resolution version with _hidpi suffix)
item_mute_off (sliced-button): (also supports 2x resolution version with _hidpi suffix)
item_mute_on (sliced-button): (also supports 2x resolution version with _hidpi suffix)
item_note_off (sliced-button): (also supports 2x resolution version with _hidpi suffix)
item_note_on (sliced-button): (also supports 2x resolution version with _hidpi suffix)
item_pooled (sliced-button): (also supports 2x resolution version with _hidpi suffix)
item_pooled_on (sliced-button): (also supports 2x resolution version with _hidpi suffix)
item_props (sliced-button): (also supports 2x resolution version with _hidpi suffix)
item_props_on (sliced-button): (also supports 2x resolution version with _hidpi suffix)
item_volknob (sliced-button): (also supports 2x resolution version with _hidpi suffix)
item_volknob_stack (knob-stack): (also supports 2x resolution version with _hidpi suffix)

There are no 150%/200% files for meter_ images, so it's showing the 100% one, much smaller than it should be.
I have not been able to test it but it seems some other images like _inline could use 150%/200% files.
Walter issues/notes

The Master track from the mixer has an alignment issue on the IO button, it's 1pixel off horizontally.
This it not an error but I think it's hard to read when meters are separated by 2 pixels and inconsistent with the MCP Master which has 1 pixel. It's that way on the TCP and on the MCP, and would likely benefit from being 1 pixel everywhere.

Here it's from my theme with a 1pixel spacing, feels more readable/clear.
Color Logic
Hoping by Reaper 7.0 we might get a look at some Default colors for envelopes for example. It seems it's been mostly unchanged since Reaper 1.0 and a lot of them are really random/lack color logic.
I don't get why Send Mute is yellow if none of the other Send envelopes are, it could be red. That being said it would make sense for Sends to be yellow since it's the color of the send from the IO button. That'd be consistent. But as of now it's mostly random.
This is mine for reference, it's not perfect, maybe you hate it, but it's definitely more consistent.
Hopefully I didn't forget something. If anything can be made on Default 6.0 about the missing images for itempool / inline that'd be great.