By Erik Kirschbaum

HANOVER, Germany (Reuters) - Germany's Greens have gone grey. The world's most successful pro-environment party has turned deadly serious about gaining power by stealing votes from Chancellor Angela Merkel - and perhaps by joining her.

The muesli, woolly sweaters, thick beards and endless debates about abstract issues that were once part of any Greens congress are largely gone. In their place is a more mature party of smartly dressed professionals with one clear aim: getting back into government after federal elections next year.

At their unusually harmonious three-day party congress in Hanover that ended on Sunday, Greens leaders were applauded for hailing their party's "conservative values" and unabashedly trying to appeal to center-right voters, using language that a decade ago would have had them booed off the stage.

Pollsters put support for the Greens at 13 percent, enough if the electoral arithmetic goes their way to make them kingmakers after Germans vote in September, 2013.

The party would prefer a coalition with the Social Democrats, renewing a government which ruled Germany from 1998 to 2005. But Greens are quietly thinking the unthinkable and opening up to a possible alliance with Merkel's conservatives, long their political arch enemy.

Greens express distaste for an alliance with Merkel and her Christian Democrats (CDU), but interest in her supporters. "We don't want the CDU, we want only your voters," Katrin Goering-Eckart, a newly-elected party leader, told the Congress.

Goering-Eckart, a Lutheran church leader, expresses the Greens' pride in their weightiness, openly admitting their hope that the makeover will attract conservative voters.

"If you want to run the country, if you want policies that add up, then you've got to be serious about it," she told Reuters. "It's not something you can do with smoke and mirrors."

The problem for the Greens is that their preferred partners, the Social Democrats (SPD), are languishing at 30 percent support in opinion polls. This may not be enough for the two parties to win a parliamentary majority and oust Merkel, whose conservatives are polling about 39 percent.

Once a peacenik ecological movement with a far-left tilt, delegates in Hanover made clear that they are no longer dead set against a coalition with Merkel, even though many prefaced their remarks with "We'd rather have an SPD-Greens government, but..."

The Greens - once famous for their unpredictable and self-destructive congress battles that could stretch beyond midnight - have already proved they can attract conservatives.

Last year they stunned Merkel by winning control of the state of Baden-Wuerttemberg, traditionally a conservative bastion, with the SPD as junior partners.

Under Winfried Kretschmann, Germany's first Green state premier, they have won a reputation as a "safe pair of hands" and extended their power when a Green became mayor of the state capital, Stuttgart, which is home to some of Germany's biggest companies including carmaker Daimler-Benz.

At a state level, they have already worked with conservatives. They ruled Hamburg with the CDU for three years until 2011 in a coalition that earned them national respectability as a fiscally responsible party.

A MORE POWERFUL FORCE

In their early years, the Greens had an aversion to power after they were founded in 1980. Only in the mid 1990s did they begin to shed their "anti-party party" ways and actively seek to be part of a federal government. As junior partners to the SPD under Chancellor Gerhard Schroeder, they helped shape policies on social and energy issues that profoundly changed Germany.

The Greens are now a much more powerful force than in 1998 when they won 6.7 percent of the vote. They got a record 10.7 percent in the 2009 election and could break that next year.

Goering-Eckart, 46, is the main reason that speculation is rampant about an alliance with conservatives - who are "black" in Germany's political color coding. A fresh-faced centrist, she was the unexpected winner of a primary battle to become the party's lead candidate, alongside veteran Juergen Trittin.

Goering-Eckart beat off two other more established women and will be the face of the party in next year's campaign. Analysts believe she can help to poach some votes from Merkel.

"Goering-Eckart is in the centre and will attract some conservative voters," said Thomas Jaeger, political scientist at Cologne University. "But the question is will the party follow her into the centre or will she move left? I think the Greens are going to have a hard time winning over lots of CDU voters."

Jaeger believes the Greens' attempt to present themselves as a more serious centrist party is an ill-fated attempt to distract from the fact that Merkel has robbed them of their most important issue - switching off nuclear power. After last year's Fukushima disaster in Japan, Merkel reversed course and decided to shut down Germany's nuclear industry.

The Greens briefly soared to highs of 24 percent in opinion polls amid fears of nuclear power. "The Greens don't really have a campaign issue any more," said Jaeger. "They can talk about social issues but who's listening to the Greens on that? They're trying to distract attention with the talk of 'black-green'."

Many delegates at the congress spent much time speaking against "black-green" as a coalition option, even though few wanted to rule it out - a subtle but significant shift from four years ago when black-green was anathema. One Greens leader said the party should keep all options open.

"If every party rules out any coalition except their preferred alliance there won't be a government and that's not good for Germany," Tuebingen mayor Boris Palmer told Reuters.

"We prefer the SPD but shouldn't rule anything. It's wrong to rule anything out before the election. 'Black-green' is not likely but shouldn't be ruled out. If the CDU makes an offer for a coalition with more green policies in it than without us, we should take it. Otherwise, we're better off in the opposition."

(editing by David Stamp)