Resize theme_name string

[bm3719]

Was getting this error running make:

cc -O2 -pipe  -c -std=c89 -Wall -Wextra -Werror -O2 `pkg-config --cflags pangocairo` settings.c
settings.c:659:49: error: 'sscanf' may overflow; destination buffer in argument 3 has size 100, but the corresponding specifier may require size 101 [-Werror,-Wfortify-source]
                if (1 == sscanf(line, " [%100[-a-zA-Z0-9]] ", theme_name)) {
                                                              ^
1 error generated.
*** Error 1 in /home/bm3719/src/c/oxbar (Makefile:38 'settings.o')

I think this is because sscanf can return up to 101 bytes (100 characters + \0).