fix: Panic caused by the absence of LastClr

[shcabin]

ECMA-376-1:2016 states that lastClr is optional
<xsd:attribute name="lastClr" type="s:ST_HexColorRGB" use="optional"/>
and example:

When lastClr does not exist, it will result in empty strings in the theme ("lt1", "dk1" ).

However, I am not sure if returning an empty string is the best option for themeColor() in this case.

[tealeg]

"Applications shall use the @lastClr attribute to determine the absolute value of the last color used if system colors are not supported." - in any case, if it simply isn't present, there's not much we can actually do about that.

[tealeg]

Basically it seems we have two problems here:

• we're round tripping style information via a mechanism that was designed to only extract a usable colour for on screen presentation, which is actually a fairly unlikely use-case for the library - in general it would be better to preserve the underlying complex colour definition rather than forcing everything to aRGB.
• we're "resorting" to lastClr because we don't have a definition of the named "system colors".

Defaulting to the empty string here is likely to have odd results for some people.

[tealeg]

See comments inline.

[tealeg]

Nice comment. I feel like we really need to make the underlying options for a colour transparent to the public interface of the library. At the moment all of this code is pointing towards a use case where we need to know a concrete aRGB value for the colour - the only case where you need that is if you're rendering the document somewhere and this seems to be a less likely use case than generating or modifying a document. In that case we'd be better off just preserving the stylistic information as it is. Sadly fixing that means creating a v4 and I don't have the time or inclination to tackle it.

[tealeg]

Given the way we intend this code to be used, the right fix for this situation is to define the values for the systemColors and use those values when lastClr is omitted.

[tealeg]

It's not clear that this won't create unexpected errors. See my comment above. The way in v3 would be to define the system colours in a lookup table and insert those values when lastClr is missing.