Colors are hard to read with a light theme

**Describe the bug**
The colors of Catch2's output do not adapt to the terminal's theme, making it unreadable when using a light theme.

**Expected behavior**
Something readable :<
Adding an environment variable to let the users invert the output would probably be the best solution. Right now I have to use the flag to disable colors but this is not desirable.

**Reproduction steps**
Use a light theme (eg solarized light) and try anything:
```cpp
#define CATCH_CONFIG_MAIN
#include "catch.hpp"

TEST_CASE( "foo", "[bar]" ) {
REQUIRE(false);
}
```

**Platform information:**

- Catch version: **v2.8.0**

**Additional context**