Creating local variation of match-theme

[Jack Hill]

Hi Guix,

I'd like to create a local variation of the matcha-theme package with one 
of the colors changed to suit my taste. Rather than make the change in my 
clone of the git repository, which could then become outdated, I thought I 
would try to make the change programatically as part of the package 
definition. I decided to make my change in a snippet as part of the origin 
specification because that seemed to logically fit with what I was trying 
to do (build a package with modified source) and because matcha-theme uses 
the trivial build system, which I don't believe has the concept of phases, 
so it wasn't clear how I would add a phase to make my change during the 
build. I came up with the following package definition:

```
(package
  (inherit matcha-theme)
  (source (origin
            (inherit (package-source matcha-theme))
            (modules '((guix build utils)))
            (snippet
              '(begin
                 (substitute* (find-files "." "\\.css$")
                   (("abb9b6") "859900"))))))
  (synopsis "jackhill's version of the matcha-theme"))
```

Unfortunately, after building my modified source tarball correctly, the 
package build fails. I believe this is because matcha-theme's 
trivial-build-system recipe expects a source checkout and not a tarball, 
and doesn't have the logic to expand the tarball that, e.g., the 
gnu-build-system provides.

The build log contains the following:

```
@ build-started 
/gnu/store/rqx5yqh4ncj02rxzfwvp9jcqv9dk8l1g-matcha-theme-2019-11-02.drv - 
x86_64-linux 
/var/log/guix/drvs/rq//x5yqh4ncj02rxzfwvp9jcqv9dk8l1g-matcha-theme-2019-11-02.drv.bz2
 28285
Backtrace:
           4 (primitive-load "/gnu/store/j01a49zzlrn7fyr2x7ibxyqsph5?")
In ice-9/eval.scm:
   191:35  3 (_ #f)
    619:8  2 (_ #(#(#(#(#(#<directory (guile-user) 7f?> ?) ?) ?) ?) ?))
In 
/gnu/store/ygivy1fvr7gbyva4z22b7vzzps1krbq5-module-import/guix/build/utils.scm:
   343:27  1 (_ "/gnu/store/ajbvkiw30mmfhj8mpjfyvnxpjl0ycj07-matcha?" ?)
In unknown file:
           0 (copy-file "/gnu/store/ajbvkiw30mmfhj8mpjfyvnxpjl0ycj0?" ?)

ERROR: In procedure copy-file:
In procedure copy-file: Is a directory
`/gnu/store/ajbvkiw30mmfhj8mpjfyvnxpjl0ycj07-matcha-theme-2019-11-02.tar.xz' -> 
`/tmp/guix-build-matcha-theme-2019-11-02.drv-0'
builder for 
`/gnu/store/rqx5yqh4ncj02rxzfwvp9jcqv9dk8l1g-matcha-theme-2019-11-02.drv' 
failed with exit code 1
@ build-failed 
/gnu/store/rqx5yqh4ncj02rxzfwvp9jcqv9dk8l1g-matcha-theme-2019-11-02.drv - 1 
builder for 
`/gnu/store/rqx5yqh4ncj02rxzfwvp9jcqv9dk8l1g-matcha-theme-2019-11-02.drv' 
failed with exit code 1
```

For reference, matcha-theme's builder is:

```
(arguments
     '(#:modules ((guix build utils))
       #:builder
       (begin
         (use-modules (guix build utils))
         (let* ((out (assoc-ref %outputs "out"))
                (source (assoc-ref %build-inputs "source"))
                (bash (assoc-ref %build-inputs "bash"))
                (coreutils (assoc-ref %build-inputs  "coreutils"))
                (themesdir (string-append out "/share/themes")))
           (setenv "PATH"
                   (string-append coreutils "/bin:"
                                  (string-append bash "/bin:")))
           (copy-recursively source (getcwd))
           (patch-shebang "Install")
           (mkdir-p themesdir)
           (invoke "./Install" "-d" themesdir)
           #t))))
```

This leaves me with two questions:

How should I accomplish what I want to do (change one of the colors in 
matcha-theme for local use)?

If I am right about trivial-build-system (that it makes it more difficult 
to create modified packages compared to other build systems), should we 
try to avoid using it in gnu/packages to ensure people have the easiest 
time exercising their software freedom?

Best,
Jack