I think this might be it since I've seen a couple of adults on the same plant that fit this species. I'll keep the nymphs well fed...
P.S. May 21, 2010: must have molted into an adult overnight, see the photo just submitted.

Hi Hartmut :-) Interesting that you found this mirid on Sphaeralcea. When I first saw this species it was on bush monkeyflower (Mimulus aurantiacus):

   

Joyce had shown me the above individual in the field...and mentioned that she associated these mirids with Mimulus aurantiacus, on which she'd seen them previously (e.g. see her comment here and CalPhotos posts here). A number of BG posts reinforce her observation:

              (see also this Flickr post)

Not too long afterwards, I saw Closterocoris amoenus again (this time with Harsi at RSABG :-)...but on Salvia:

   

Other BG posts show it on Salvia clevelandii, Lepechinia, a(nother) mallow, Urtica...and a number of other (unidentified) plant species:

                    ...            

The above BG posts suggest Closterocoris amoenus may not be especially picky regarding the plants it associates with...beyond possibly an partial affinity for Mimulus aurantiacus or various members of the mint and mallow families?

But I'm curious whether any info on particular host preferences might be buried in the literature, or known as "folklore" among mirid workers? I checked all the references cited by Joyce here...none had any plant host info for Closterocoris, though the there were 4 pages here with nearly 200 Great Basin plant species (including "Diplicus longiflorus = Mimulus aurantiacus!) and hundreds of associated mirids.

Looking at my records, they were seen in my garden on Sphaeralcea sulphurea (native to Guadalupe Island, Mexico), and on Salvia 'Allen Chickering'. I have no other records.

I went back through my photo archives to see if I could find any more data for you. Unfortunately, I only had a few more records in addition to that one we found together on Salvia. Here's what else I have:

Adult on Cucurbita palmata at Nix Nature Center in Orange County on May 29, 2010.
Adult on a thistle species (Cirsium maybe?) from my old place in Webb Canyon in Los Angeles County on May 22, 2011.

Hi Harsi & Hartmut. A rather belated response here, but recently I've been trying again to get a better handle on plant associations for this mirid, and thought I'd share my observations.

A number of "hosts" are listed on this "Discover Life" web page for Closterocoris amoenus...a resource I missed earlier (and just added to the species info page). That "DL" host plant list appears to have been compiled from label info on museum specimens. It's often hard to know when "host plant status" (e.g. obligate, or at least consistently preferred, larval or adult use of a food plant) has been carefully verified, vs. when a insect has simply been observed on the given plant taxon (e.g. an "accidental visit"). So I'm using "plant associate" to refer to any plant the bug has been seen on (preferably repeatedly).

Apropos to your comments above: various Salvia, a Sphaeralcea, and Cirsium occidentale are listed on the "Discover Life" page (and I found a single iNat post that appears to be on a thistle). Harsi's observation on Cucurbita palmata is especially intriguing...as no cucurbit appears on the "Discover Life" list, and I couldn't recognize any in perusing iNat observations.

Among the many current observations of C. amoenus on iNat there are lots showing Closterocoris on common bush monkeyflower (Diplicus [Mimulus] aurantiacus), various species of Salvia, and various members of Malvaceae...but I was also impressed by the many iNat posts on other plant genera & families both on the "Discover Life" list and missing from it.

Below is a (partial) list of associated plant taxa for C. amoenus gleaned from studying iNat & BugGuide posts, organized by family:

Phyrmaceae: Common bush (or "Sticky") monkeyflower (Diplicus aurantiacus).
Malvaceae: In addition to Sphaeralcea mentioned earlier, a Lavatera appears on iNat, and Malacothamnus appear among BG & iNat photos (e.g. BG 2109194, iNat 78910541, iNat 111997471, and others), though this genus isn't currently on the DL list.
Lamiaceae: In addition to the Salvia & Lepechinia mentioned earlier, there appears to be a photo here on Monardella villosa.
Asteraceae: In addition to the Cirsium mentioned earlier, there appears to be lots(!) of iNat observations on the southern California species Bahiopsis lacinata appearing on the DL list. I didn't recognize any iNat posts on the other two Astercaceae species on the DL list, but there are plenty of observations on other members of the family (e.g. a 25025962, 24213732, 24332351, and many more).
Hydrophyllaceae: The "Discover Life" list includes some species of Eriodictyon:...but not E. tomentosum, as seen here.
Phacelia distans is on the DL list (some of these may be P. distans: 112075721, 20287165, 11690149, 110466646, and 110141297); and, additionally, P. californica appears on iNat.
Fabaceae: The current DL list includes only Cystisus. Additionally, Lupinus appears in this BG post and a number of iNat observations (e.g. 112018760, 120105375, 112304157, 120105436, and 6118376)...as well as what looks like possibly an Astragalus?
Families currently missing from the DL list are:

Convolvulaceae: Various morning glories (Calystegia, etc) appear in BugGuide posts (e.g. BG 1814570) and iNat observations (163827190, 25353296, 23712140).
Papaveraceae: There are iNat observations on Californica poppy (153650149, 156462726).
Onagraceae: There are iNat observations on Camissonia relatives (156461958, 158883367), and a Clarkia.
Rosaceae: There's an iNat observation on a rose species (160650397).
Gentianaceae: There's an iNat observation on a Zeltnera.
Cleomaceae: There are two iNat observations (164573471, 164911648) on "Bladderpod" (Peritoma arborea )...somewhat surprising considering the rather repulsive-smelling herbage of that species.
Asperagaceae: There are iNat observations on what look like monocot "Blue Dicks" relatives (109987887, 111856463).
Most likely some of the above are "accidental" plant associations (e.g. just "exploring", or...?) rather than true "(larval) host plants"...but this mirid does not seem tightly associated with a single host plant family!

[Unintentional comment submission snafu here (and below)...made while editing the comment above. Unfortunately, can't delete these on BugGuide!]

Ditto above

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I think this might be it since I've seen a couple on the same plant. I'll keep the nymphs well fed...